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Effects of Mollusk Size on Growth and Color of Cultured Half-Pearls from Phuket, Thailand

Authors:
  • Prince of Songkla University - Phuket
Most of the pearl farms in Phuket, Thailand,
use Pteria penguin (Röding, 1798) to pro-
duce half-pearls (Kanjanachatree et al.,
2003, 2019). These typically have a cream and white
color (figure 1), but the mollusk also produces yellow
pearls, black half-pearls, and bicolor half-pearls
(Shor, 2007). The major determining factor in the vi-
ability of the cultured half-pearl industry is consis-
tent production of good-quality half-pearls as
demanded by the market (Ky et al., 2014a,b). Re-
search is needed to improve quality and consistency.
In addition to size, thickness, and weight, consider-
ation should also be given to the luster, darkness,
shape, and visible color. The color of pearls is defined
by two main criteria: the overall bodycolor of the
pearl, a combination of many pigments; and second-
ary color that is due to refraction and wavelength in-
terference effects, with some colors scattered on the
pearl surface (Karampelas et al., 2011; Abduriyim,
2018). Some pearls are bicolor (white and yellow or
black and yellow) (Kanjanachatree et al., 2019). The
factors that contribute to different colors are the
mollusk species, pearl thickness, water quality of
the pearl farm, plankton density (food quality and
quantity) (Snow et al., 2004; Shor, 2007), and the ge-
netics of the mollusk population (Ky et al., 2014a,b,
2018).
Plankton are small organisms that float freely in
the water. Almost all types of algae, including some
bacteria and molds, are also present. Zooplankton
consist of single- and multi-celled organisms, includ-
ing small invertebrates, and also include the larvae
of fish and invertebrates. Phytoplankton are small
single-celled or multi-celled or colonial algae. They
are the most abundant of all plankton and the major
EFFECTS OF MOLLUSK SIZE ON GROWTH
AND COLOR OF CULTURED HALF-PEARLS
FROM PHUKET, THAILAND
Kannika Kanjanachatree, Napapit Limsathapornkul, Amorn Inthonjaroen, and Raymond J. Ritchie
FEATURE AR ICLES
The pearl mollusk Pteria penguin is popular for the production of half-pearls (mabe or cultured blister pearls).
Research was carried out on two shell size classes implanted with half-pearl nuclei: the standard 140–145 mm
size class (n = 200) was the control, and the smaller 130–135 mm size class (n = 200) was the test group. The
mollusks were hung on strings at a sea depth of 2 m for up to 10 months. Implant rejection rates for the two
mollusk groups were not significantly different, but the small mollusks had much lower mortality and grew
faster. Nacre growth was not significantly different. Digital spectral analysis showed that the three red-green-
blue (RGB) colors of the cultured half-pearls followed independent sinusoidal seasonal patterns with time periods
of about 10 ± 0.3 months. The amplitude of variation in the RGB colors of the cultured half-pearls were all
larger in the small pearl mollusks.
In Brief
• Color development of half pearls in Pteria penguin in
small and standard-sized mollusks was followed over
10 months. The smaller mollusks grew faster and had
better nacre growth.
• RBG (red-green-blue) color analysis showed that the
colors of the half-pearls varied during development fol-
lowing sine curves.
• The sinusoidal curves of red, green, and blue colors
were out of phase, and so the overall color varied with
age and size class of the host mollusk.
• The periodicity of color development has important
consequences for color-matching of half-pearls.
See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY
, Vol. 55, No. 3, pp. 388–397,
http://dx.doi.org/10.5741/GEMS.55.3.388
© 2019 Gemological Institute of America
388 PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019
Figure 1. Cultured half-
pearls from different-
sized Pteria
penguin mollusks.
Photo by Kannika Kan-
janachatree.
photosynthetic primary producers in marine ecosys-
tems (Falkowski and Raven, 2007). They float in the
water freely, but there are some mobile species with
flagella. Although some are large enough to see with
the naked eye, most are well below this in size. Phy-
toplankton use solar energy to fix inorganic carbon
and convert nutrients. The primary photosynthetic
pigment, chlorophyll a, is routinely measured to in-
dicate the biomass of phytoplankton in water
(Falkowski and Raven, 2007; Kanjanachatree et al.,
2019).
The purpose of this research was to find ways to
increase the overall production and value of cultured
half-pearls. We set out to determine the optimum
mollusk size and compare the rejection rate, the
mortality rate of the seeded mollusks, and the color
and quality of the cultured half-pearl, particularly
any changes in the quality of the color from month
to month in the cultivation of Pteria penguin.
MATERIALS AND METHODS
Collection and Sorting of Mollusks. A batch of more
than 400 wild mollusks were collected from the
Sapum Bay area of Phuket, near the pearl farm, in Au-
gust 2015. In Phuket, this is the middle of the wet
season. They were measured for overall length and di-
vided into two classes. Based on the breeding seasons
for Pteria penguin, these young mollusks were prob-
ably all 2–2½ years of age (Milione and Southgate,
2012; Kanjanachatree et al., 2019).
Of these, 200 pearl mollusks were designated the
large or standard mollusk class, measuring 140–145
mm in length. Because this is the size class typically
used for half-pearl cultivation, we made this the con-
trolgroup (figure 2). The secondclass consisted of 200
smaller mollusks, measuring 130–135 mm in length.
These formed the test group for the experiment.
Implantation Procedure. Half-pearl nuclei (17 mm di-
ameter) were implanted in August 2015, and the ex-
periment was run for 10 months. Standard half-pearl
cultivation practices were followed as described by
Taylor and Strack (2008) and in our previous publi-
cations (Kanjanachatree et al., 2003, 2019). The mol-
lusk was left in the air for 30 minutes, or in
Figure 2. Different size classes of Pteria penguin pearl
mollusks: small (130–135 mm) and standard (140–
145 mm). Photo by Kannika Kanjanachatree.
PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019 389
Figure 3. Drilling shells for hanging mollusks on strings: a pearl mollusk prepared for drilling (left), the drilling pro-
cedure (center), and the stringing (right). Photos by Kannika Kanjanachatree.
continuously running seawater. Shells began to open,
and then a small wooden wedge was inserted into the
mollusk to hold it open (Shor, 2007). Shells were
placed in a clamp, pliers were used to insert the
speculum, and the mollusk was held open using a
metal spatula. The nucleus is made from resin and
has a base length of 17 mm. It is attached to the shell
with a glue containing ethyl-2-cyanoacrylate. Nuclei
were placed nearthe adductor muscle in the standard
position used for mabe (cultured blister) half-pearls.
A small hole was drilled in each shell so the mol-
lusks could be hung in the water in strings, two me-
ters below the surface to avoid excessive sunlight
(figures 3 and 4). Mollusks also receive a larger
amount of plankton at this depth (Kanjanachatree et
al., 2003). All mollusks were checked each month
and scrubbed to remove fouling organisms (Taylor
and Strack, 2008). Harsher methods for removing
fouling organisms were avoided.
Assessment of Growth of Cultured Half-Pearls, Food
Resources, and Half-Pearl Quality. Ten mollusks
were randomly selected each month for assessment.
These mollusks were removed from the experiment,
Figure 4. Preparation
and hanging the mol-
lusks on strings two
meters below the
water’s surface on the
floating raft pearl farm.
Photo by Kannika Kan-
janachatree.
390 PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019
so the cohort decreased from 200 at month 0 to 100
at month 10. In our previous study (Kanjanachatree
et al., 2019) the parameters could be measured non-
destructively and the mollusks were returned to the
water, but the measurements made in the present
study necessarily involved the death of the mollusks.
The assessed parameters were mortality, rejection
of nucleus by the mollusk, half-pearl nacre forma-
tion, pearl nacre thickness, and growth of shell from
the differently sized Pteria penguin mollusks. Data
were recorded over the 10 months of the project
(mean ± 95% confidence limits, n = 10).
Chlorophyll Analysis. Chlorophyll measurements
were made on water samples taken for the environ-
mental monitoring. The water samples were col-
lected from the pearl farm at four points to determine
the average chlorophyll and carotene content. Cells
were filtered using standard 0.45 μm filter disks.
Chlorophyll was assayed using a formula for a mixed
phytoplankton population (Ritchie, 2006) using
ethanol solvent and a Shimadzu UV-1601 UV-visible
dual beam spectrophotometer. Water sample vol-
umes were 500 mL. Chlorophyll content was ex-
pressed as μg/L ofwater sample. The monthly results
(see supplementary table 1 at https://www.gia.edu/
doc/FA19-Phuket-Half-Pearls-Supplementary-
Table1.pdf) were similar to those published previ-
ously in Kanjanachatree et al. (2019).
Color of Cultured Half-Pearls. There has been a
resurgence of interest in quantitative color analysis
of pearls (Ky et al., 2014a,b, 2018), particularly as a
result of the finding that selective breeding for color
is possible. The color of the half-pearls was measured
using the method and simple apparatus described by
Choodum et al. (2014) for use on drug samples. The
device uses readily accessible mobile phone technol-
ogy for digital image analysis of red, green, and blue
(RGB) light. The device is a simple light box fitted
with a white LED light source and constructed using
minimal equipment, as opposed to an integrating
sphere scanning spectrophotometer that is highly
specialized and not readily available. The measured
colors of the half-pearls were compared month to
month and compared to a set of cultured half-pearl
standard colors (Phuket Pearl Industry Standards).
Statistical Analysis. Completely randomized design
data were analyzed by one-way analysis of variance
(ANOVA), and comparisons of mean values were
made using Duncan’s new multiple range test
(DMRT) using standard statistical analysis packages.
The confidence level of p < 5% was chosen as the sig-
nificant difference criterion. Samples with the same
superscript letter were not significantly different (p
< 0.05). Standard linear regression methods were used
to fit linear regressions (Y = mx + b) to data. The sea-
sonal sinusoidal fit to the half-pearl color data was
performed using non-linear least squares methods,
and the asymptotic errors of the fitted parameters
were calculated by matrix inversion. The standard
statistical reference text used was Snedecor and
Cochran (1980).
RESULTS AND DISCUSSION
Water Quality and Chlorophyll Content Over the
Course of This Study. Phuket has a wet maritime
monsoonal climate, with a wet season from April to
November each year (about 300 mm/month) and a
dry season the rest of the year (about 100
mm/month) (Kanjanachatree et al., 2019). The dry
season drought is therefore not as severe as in main-
land Asia. The study was begun in August and con-
tinued over 10 months, so it extended from the
middle of the wet season to the beginning of the next
wet season. During the monthly monitoring of the
mollusks, midday measurements were made of Sec-
chi depth (cm), air temperature (°C), surface water
temperature (°C), salinity in parts per thousand (‰),
and pH using standard methods for measurements of
water and wastewater (Cleseri et al., 1998, known as
the APHA manual). Similar monthly results have
been published previously in Kanjanachatree et al.
(2019). Overall, the results in the present study were
very uniform, with little seasonality in the five pa-
rameters of water quality just mentioned, so overall
values were calculated (see supplementary table 2 at
https://www.gia.edu/doc/FA19-Phuket-Half-Pearls-
Supplementary-Table2.pdf). The calculated values
compared well with previous environmental data for
the study site (Kanjanachatree et al., 2019). The Sec-
chi disk depth was 106 ± 12 cm, the water tempera-
ture was 29 ± 0.8°C (range of 26–32°C), the salinity
was 30 ± 0.5 ppt (range of 27–35 ppt), and the dis-
solved oxygen was typically high (5.63 ± 0.64 mg/L
87% saturation, range of 5.2–7.3), and the overall pH
was typical for seawater (8.3). Seawater becomes
more acidic when CO2levels are high and more al-
kaline when phytoplankton remove CO2. There was
very little seasonality of conditions at the study site
compared to other pearl-growing areas in more tem-
PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019 391
+ + + + + +
PEARL IMPLANT LOSSES OVER TIME (MONTHS)
PERCENTAGE OF MOLLUSKS (%)
0
40
50
10
60
20
30
80
90
100
70
Rejection % Control Cumulative
Rejection % Small Cumulative
Mortality % Control Cumulative
Mortality % Small Cumulative
% Total Losses Control Cumulative
% Total Losses Small Cumulative
0123456789
MONTH (AUGUST IS MONTH 0)
perate climates such as Japan (Tomaru et al., 2002a,b;
Muhammad et al., 2017), Mexico (Ruiz-Rubio et al.,
2006), and Australia (Milione and Southgate, 2012).
The chlorophyll a levels in the water column
were remarkably constant over the growing period
used in this study, about 0.8 μg/L (0.868 ± 0.046) ex-
cept for a January high level of about 1.0 μg/L (1.02 ±
0.079). Overall mean values and ± 95% confidence
limits have been calculated (again, see supplemen-
tary table 1). The relative amounts of chlorophylls a,
b, and c are a guide to the predominant types of
plankton present over the year (Falkowski and
Raven, 2007): Chlorophyll b indicates the presence
of green algae (Chl a +b), and chlorophyll c indicates
the predominance of chromo flagellates and diatoms.
High carotenoids indicate a heavy presence of Chl a
+ c organisms. By such criteria, the relative abun-
dance of algae containing chlorophyll b was more or
less constant over the year, but there were high levels
of Chl c and carotenoids in October (month 2) at the
end of the wet season and in December (month 4),
which is in the first half of the dry season. Large algal
blooms did not occur during the course of the study,
and the plankton food supply for the mollusks was
remarkably constant, reflecting Phuket’s tropical
maritime climate.
Figure 5. Rejection,
mortality, and total
losses of the control
group and the test
group after implanta-
tion of a half-pearl nu-
cleus over time. The
control mollusks had
higher mortality rates
and overall losses than
the small mollusks.
Performance of the Different-Sized Pteria penguin
Pearl Mollusks. The mollusks used in the study
would have been about 2 to 2½ yearsold (Milione and
Southgate, 2012; Kanjanachatree et al., 2019).Smaller
pearl mollusks had better growth, much lower mor-
tality, and lower overall losses than the larger mol-
lusks in the control group (supplementary table 2;
figure 5). The first-month rejection rates, mortality
rates, and total losses for the control group and the
small mollusks were not significantly different (rejec-
tion 7.3 ± 3.6%; mortality 6.0 ± 3.3%; overall losses
13.3 ± 4.7%, n = 400). The 10-month rejection rate
(16% vs. 13%) was not significantly different, either,
but there wasalarge difference in the mortality rate
(39% vs. 20% respectively, p < 0.001). The sum of re-
jection + mortality was consequently much higher in
the control group than in the small mollusks (55%
vs. 33% respectively, p <0.001). After the losses in the
first month, the rejection rate, mortality, and overall
losses were approximately linear over time (p <<
0.001), with no obvious seasonal effect (figure 5). After
the first month, the rejection rate was not signifi-
cantly different for the two mollusk sizes, and so the
overall mean rejection rate was only about 0.606 ±
0.151%per month (r = 0.8924). The mortality rate and
hence overall loss (above) were much higher for the
392 P
RODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN
G
EMS & GEMOLOGY
F
ALL 2019
10
control mollusks (3.575 ± 0.340% per monthvs. 1.490
± 0.364% per month), resulting in a total loss rate of
4.288 ± 0.389% per month (r = 0.9939) for the control
mollusks vs. 1.989 ± 0.544% per month (r = 0.9482)
for the small mollusks. The rejection rates, mortali-
ties, and total losses for the control mollusks in this
study are comparable to those found previously in
Phuket pearl mollusks (Kanjanachatree et al.,
2018a,b). Growth rates of shell and half-pearls were
determined by linear regression (supplementary table
2; figure 5). The shells of the small mollusks grew
faster (3.274 ± 0.177 vs. 2.231 ± 0.927 mm/month,
meanvalues ± 95% confidence limits), but their pearl
nacre did not (0.3134 ± 0.05334 vs. 0.2458 ± 0.1349
mm/month). The standard-sized mollusks routinely
used for half-pearl farming using Pteria penguin are
actually suboptimal for shell growth, though not for
nacre growth of the half-pearls, but their mortality
rate is unacceptably high.
Feeding. Tomaru et al. (2002a,b) studied the relation-
ship between akoya pearl mollusks and chlorophyll
a concentration in phytopigment extracts of the wet
mollusk tissue (basically a measure of the stomach
contents) and the dry tissue weight in the pearl mol-
lusks at Ago Bay (1967–1969) and Ohmura Bay
(1984–1985) in Japan. Their study showed that the
concentration of chlorophyll a as a measure of the
mollusks’ diet affected their growth and growth of
the implanted pearls. However, Japan has a temper-
ate climate with four distinct seasons and a spring
plankton bloom, and so plankton availability was
much more seasonal than in Phuket. The dry season
in Phuket is not a period of extreme drought as it is
in many monsoonal climates: There is at least some
rain all year.
Pearl mollusks feed by filtration, either of live
food or dead food. Sometimes they absorb dissolved
organic matter (DOM), but the importance of this as
a source of nutrition has not been well documented
or quantified. Very small plankton (picoplankton, ≈1
μm) are very important to pearl mollusks. They feed
on other picoplankton and nanoplankton much
smaller than the effective mesh size of their filtration
apparatus because their gills are covered in a sticky
mucus that traps nanoplankton (Tomaru et al.,
2002a). Bacteriastrum, Leptocylindrus, Melosira,
Nitzschia, Rhizoslovenia, Skeletonema, Thalas-
sionema, and Thalassiosira are all consumed by the
mollusks (Martinez-Fernandez et al., 2006). Some of
these species were found at the Phuket pearl farm,
but the more common species were the diatoms
Chaetoceros and Skeletonema, which are generally
regarded as non-toxic. Some species such as the
Nitzschia species are also blamed for shellfish food
poisoning and mortality. Where Nitzschia were
found in large quantities and for a prolonged period
of time, the mortality rate of the shellfish was found
to increase. Tomaru et al. (2002a) reported that the
Nitzschia species that bloomed in a bay in Japan’s
Uchiumi Prefecture caused the death of akoya pearl
mollusks in 1998. Nitzschia contains chlorophyll a
+b, so such blooms would be indicated by very high
levels of Chl a +b, which did not occur at the Phuket
pearl farm during this study (Kanjanachatree et al.,
2018a,b). Similarly, other harmful blooms are often
caused by dinoflagellates, which have high levels of
chlorophyll a + c and high carotenoids. See Tun
(2000), Shor (2007), and Taylor and Strack (2008) for
overviews of mass mortalities in pearl mollusks and
their consequences.
Temperature. The present study measured the water
quality of the pearl farm, which was within the stan-
dard for aquaculture in Thailand. Conditions on the
farm, with a temperature range of 26–32°C, do not
cause death of the mollusks (Kanjanachatree et al.,
2019). This is consistent with the study of Tomaru et
al. (2002a,b) on Japanese pearl farms that have a tem-
perate climate. At temperatures above 20°C, food
availability and metabolism increase and have posi-
tiveeffects on shell thickness, but this does not affect
the size of the shells. Yukihira et al. (2000) reported
that at temperatures below 12°C, akoya pearl mol-
lusks (Pinctada fucata) in Nagoya have reduced nacre
secretion, which affects the production of pearls and
reduces theweight of the shellfish.But at either 7.5°C
or 35°C,the mollusks ate less. Eventually, excessively
cold or hot temperatures stop both the movement of
the cilia and the heartbeat of mollusk larvae. But the
response to temperature depends on the stage of the
larva. High temperatures make mollusks more vul-
nerable to disease by increasing their stress levels,
making them less able to cope with infections (Man-
nion, 1983).
Mollusks are ectothermic animals, so the ambi-
ent temperature affects their metabolism and sur-
vival rate. Metabolism affects respiratory processes,
absorption, excretion, and growth of tissues, as well
as reproductive cells and vulnerability to diseases.
Higher temperatures are not always good; usually
there is an optimum temperature, and higher tem-
peratures are progressively harmful. High tempera-
tures can destroy enzymes (Yukihira et al., 2000) and
PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019 393
promote diseases in pearl mollusks (Mannion, 1983).
High-temperature fatalities in aquatic organisms are
often a consequence of oxygen stress because O2sol-
ubility decreases with temperature (Yamamoto et al.,
1999; Yukihira et al., 2000). Water temperatures at
the Phuket pearl farm are exceptionally stable
throughout the year, with no sudden cold or hot
events (Kanjanachatree et al., 2019). Temperatures of
7.5°C and 35°C (Yukihira et al., 2000) are both out-
side the range found at the Phuket pearl farm.
Fouling of cultivated pearl mollusks is an expen-
sive nuisance (Dharmaraj et al., 1987; O’Connor and
Newman, 2001; Guenther et al., 2006). In the present
study, a monthly scrubbing was sufficient to prevent
overgrowth by fouling organisms. Salinity changes are
often used to control nuisance fouling organisms.
Salinities outside the normal seawater range are gen-
erally fatal. In astudy by Dharmaraj et al. (1987), pearl
mollusks at Veppalodai, India (Gulf of Mannar), had
100% mortality at salinities of 14‰, 55‰, and 58‰.
The mollusks were resistant to salinity ranges of 17–
45‰,had a mortality rate of 0%, and could withstand
fatal salinities for very brief periods by tightly closing
their shells. Encrustations of fouling invertebratesand
unwanted algae can be killed by soaking akoya mol-
lusks in fresh water for 15 minutes. Brines (saltwater
60‰) can be used to kill Polydora ciliates and tube-
worms (O’Connor and Newman, 2001). The mollusks
lack access to oxygen when their shell lid is tightly
closed. Pinctada sugillata from the Gulf of Mannar
(India) are resistant to low oxygen levels for up to 24
27 hours (Dharmaraj et al., 1987). Low-oxygen stress
leads to glycolytic acid accumulation in tissues. Ya-
mamoto et al. (1999) reported that low oxygen levels
in rapidly growing plankton blooms encourage the
growth of dinoflagellates. The pearl farm in their study
had a dense planting of mollusks. There were high
suspended solids after heavy rain, some minor pollu-
tion, and limited circulation in deeper water. Water
temperatures at the Phuket pearl farm are sometimes
as high as >32°C (Kanjanachatree et al., 2019), but
mass mortalities are rare in Phuket and similar locales
(Tun, 2000). All of the factors mentioned above can re-
sult in reduced oxygen levels,leadingto physiological
stress. Extreme treatments to control fouling are best
avoided if possible because they are likely to interfere
with shell growth and nacre formation. The overall
loss rate of Pteria penguin mollusks with implanted
half-pearls was already high (figure 5). Any activities
that might increase mortality are best avoided since
they would make cultivation uneconomical (Kan-
janachatree et al., 2019).
Phuket has two tidal cycles a day, so there is a
continuous flow of water at the pearl farm. But be-
cause it is a floating raft, the depth at which the mol-
lusks were suspended was constant. The farm is
located in a strait behind a barrier island and has a
very favorable circulation pattern, so the environ-
mental conditions there are never unfavorable (Kan-
janachatree et al., 2019) and mass mortalities are rare.
The water quality at the pearl farm easily met the
physical and chemical standards for aquaculture in
Thailand (Yukihira et al., 2006). The water currents
allowed the pearl mollusks to receive oxygen and fil-
ter feed. During the breeding season, the currents
also help spread the reproductive cells and larvae of
the shellfish. But during the low tide, the water car-
ried by currents from the coastline was turbid, which
may cause sediment to accumulate on the shell. The
decrease in plankton and oxygen levels at low tide
can cause the mollusk to become temporarily de-
prived of oxygen (Condie et al., 2006), but this was
not a problem at the Phuket pearl farm, which is in
a well-circulated site and located on a floating raft in
about 10 m of water: The farm has never had a mass
mortality event (Kanjanachatree et al., 2019). Poor
growth conditions seem to increase pearl secretion,
resulting in a faster pearl coating, but with lower pro-
duction quality (Condie et al., 2006). In addition,
there are tidal effects on the diet of P. margaritifera
in more marginal farm locations (such as enclosed
bays) because it does not tolerate high suspended
solids in water, whereas P. maxima is more resistant
to turbidity (Yukihira et al., 2006).
Color Analysis of Cultured Half-Pearls. Supplemen-
tary table 3 at https://www.gia.edu/doc/FA19-
Phuket-Half-Pearls-Supplementary-Table3.pdf
shows the color indices of the cultured half-pearls
over the course of the study, and the data is plotted
in figures 5 and 6. The RGB indices showed no sig-
nificant linear increase or decrease over time (linear
regression: y = mx + b, p >>0.05). However, there was
a noticeable sinusoidal periodic difference in color
over time, with maxima at months 4 and 5 (Decem-
ber and January) after the start of the experiment (fig-
ures 6 and 7). The effect on the blue indices is the
most noticeable, with the blue indices higher in De-
cember and January at the beginning of the dry sea-
son. The % reflectance appears to go up in the dry
season and goes down in the wet season in a sea-
sonal pattern.
A complex sinusoidal model was fitted to the
RGB data by the equation:
394 PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019
Figure 6. Cyclic nature of red-green-blue (RGB) color
properties of cultured half-pearls from the control
group over the course of the project. The mollusks
were implanted in August, month 0.
RGB COLOR OF
CONTROL HALF-PEARLS VS. TIME
0
20
60
100
140
40
80
120
160
180
200
PERCENTAGE OF STANDARD (%)
Red control
Red control ÿt
Green control
Green control ÿt
Blue control
Blue control ÿt
0 1 2 3 4 5 6 7 8 9
TIME (MONTHS, AUGUST IS 0)
Y = A × Sin (ϖ.T + D) + C (1)
Figure 7. Sinusoidal character of RGB color properties
of the cultured half-pearls grown in small mollusks.
The cyclic nature of the color of the cultured half-
pearls is more apparent in the small pearl mollusks.
RGB COLOR OF HALF-PEARLS FROM
SMALL MOLLUSKS VS. TIME
0
20
60
100
140
40
80
120
160
180
200
PERCENTAGE OF STANDARD (%)
0 1 2 3 4 5 6 7 8 9
TIME (MONTHS, AUGUST IS 0)
2π = ϖ.Tp + D
2π - D
Tp = ω
PRODUCTION OF CULTURED HALF-PEARLS FROM PTE IA PENGUIN G EMS & GEMOLOGY F ALL 2019 395
The period for this function is:
where Y is the % reflectance compared to an industry
standard, A is an amplitude scaling factor for the sine
function, T is time in months, ω is the period scaling
factor (ω = 0.5236 for T in months), D is a displace-
ment factor for adjusting where to start the sine func-
tion, C is a scaling constant (average Y over the time
period), and Tp is the repeat period of the sine func-
tion (in months). Parameters A, ω,D, and C for Equa-
tion 1 could be estimated using Microsoft Excel’s
Solver tool, and their asymptotic errors could be es-
timated by matrix inversion (Snedecor and Cochran,
1980).
Figures 6 and 7 show mean values ± 95% confi-
dence limits of standardized reflectance in half-pearls
of standard- and small-sized mollusks over the
course of the 10-month experiment starting in Au-
gust 2015. The curve fits were all highly statistically
significant (p < 0.001), and all means of the fitted pa-
rameters were significantly different from zero (see
analysis in supplementary table 1). The period scal-
ing factor (ω) would have been 0.5236 if the period
were exactly one year: The experimentally deter-
mined ω values were all greater than ω = 0.5236.
Only one ω value was significantly different from the
others (red, control), and since the Tp value was not
different, an overall value for ω = 0.8430 ± 0.0284
could be calculated. The amplitude (A) of the sea-
sonal effect was greatest in the case of the blue color
of cultured half-pearls from small mollusks and least
in the case of the green color index of half-pearls from
the control group. The amplitude (A) was noticeably
higher in the cultured half-pearls from the small mol-
lusks for all RBG colors (figures 6 and 7; supplemen-
tary table 1). The range in color was nearly ±30% in
the case of the blue color of the half-pearls from the
small group. The mean color was measured by the
constant parameter (C). As would be expected, the
average % colors for red, green, and blue were signif-
icantly different, but the three colors were not signif-
icantly different when the control group and the
small mollusks were compared as pairs. The dis-
placement parameter (D) shifted the sine curve time-
wise to optimize the fit. On an annual scale, 0.5236
radians is equivalent to one month, and so D was
equivalent to –2 to –5 months in most cases. If the
changes in color over time were synchronous, the pa-
rameter D would not be significantly different for the
different RGB colors: Supplementary table 2 shows
that D is significantly different even though the pe-
10
10
riod parameter ω was only marginally significant and
the time period (Tp) of the sine curves was not sig-
nificantly different. The RGB color of the cultured
half-pearls thus shows an asynchronous seasonality
(figures 6 and 7). Growth of the half-pearls was linear
over time (supplementary table 2). The available phy-
toplankton also appeared to be more or less constant,
with no strong seasonality as would be found in a
temperate climate (Yukihira et al., 2000, 2006;
Tomaru et al., 2002a,b).
These color trends are not readily noticeable to
human color matchers of pearls, but some matchers
do note that there appears to be a relationship be-
tween harvest time and color. Comparison of figures
6 and 7 (supplementary table 1) shows that the sea-
sonal sinusoidal effect is more obvious in the cul-
tured half-pearl from the small mollusks (figure 7)
than in the standard-sized group (figure 6). The sea-
sonal effect is least apparent in green light (human
eyesight happens to be most sensitive to green light).
These findings are important considerations in color-
matching of the pearls (Shor, 2007). The seasonal ef-
fect is nearly ±30% of standardized reflectance in the
case of blue color, and so would be important in color
matching and in displaying the pearls for sale. Mod-
ern “white” light diodes have a very heavy blue com-
ponent compared to green and red light and so are
not well color balanced compared to natural sunlight
or other common light sources. Hence, display cases
using “white” diodes will cause differences in blue
light reflectance to be more conspicuous than, for ex-
ample, under typical fluorescent lighting. More so-
phisticated color analysis using integrating sphere
technology is needed to resolve color issues of pearls,
following the work of Karampelas et al. (2011) on ma-
rine pearl mollusks and Abduriyim (2018) on fresh-
water pearl mollusks.
CONCLUSIONS
The rate of nacre deposition on half-pearl nucleus is
not significantly different between small and large
Pteria penguin mollusks, but there are significant
RGB color differences (supplementary table 1). The
growth of the mollusk contributes to the rate of half-
pearl nacre deposition. In this study, the small mol-
lusks grew faster and had a much better survival rate
than the larger ones that are the industry standard (fig-
ure 5). To achieve the highest quality, the half-pearls
should be harvested in the seventh month because the
thickness of the nucleus plus pearl layer is up to 19.4
mm (nacre 1.2 mm thick), which is the optimum for
buttons and for button-shaped jewelry. Longer incu-
bations produce pearls of increasingly uneven shape,
which are less useful for buttons and jewelry.
The RGB color of the cultured half-pearls seems
to be little affected by the month if a simple linear
regression is fitted to the data. However, more so-
phisticated analysis shows that there is a seasonal (si-
nusoidal) effect on color reflectance, particularly in
blue light (figures 6 and 7; supplementary table 1).
The smaller mollusks produced better-quality cul-
tured half-pearls and had a lower mortality and rejec-
tion rate. However, the RGB colors of the cultured
half-pearls had larger amplitudes of color variation
over the incubation period than the colors in the
larger pearl mollusks. The phases of the sinusoidal
changes in RBG colors of the half-pearls are not in-
phase (synchronous), so the overall color of the pearls
changes over the year. This has important conse-
quences for color matching.
ABOUT THE AUTHORS
Ms. Kanjanachatree is an assistant professor, Ms. Limsatha-
pornkul is a technical officer, and Dr. Ritchie is an associate pro-
fessor, at Prince of Songkla University in Phuket, Thailand. Mr.
Inthonjaroen is director of at Phuket Pearl Industry, Co. Ltd.
ACKNOWLEDGMENTS
The authors wish to thank Phuket Pearl Industry Co. Ltd. for
providing access to the pearl farm facilities. Prince of Songkla
University in Phuket provided access to equipment for analyses
done as part of the study.
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