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2NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2010
The value of a pearl is strongly dependent on its natu
ral or cultured origin (for the exact definitions of nat
ural and cultured pearls, see CIBJO, 2010). There are
two major categories of cultured pearls: beaded (bead with a
mantle-tissue graft; BCPs) and non-beaded (solely a mantle-
tissue graft; NBCPs). (For more information regarding graft-
ing and beading, see Sturman [2009] and references therein.)
Traditional X-radiographs are by far the most useful
tool to separate cultured from natural pearls (Webster,
1994). Radiographs provide a projection on a plane of the X-
ray transparency of the investigated object; typically, the
bead or structures related to the tissue used to stimulate
growth of the cultured pearl will have a different appear-
ance from that of the pearl itself. In the last decade, howev-
er, the market has received large quantities of freshwater as
well as some saltwater NBCPs that are sometimes difficult
to identify using radiography (Scarratt et al., 2000;
Akamatsu et al., 2001; Hänni, 2006; Sturman and Al-
Attawi, 2006; Sturman, 2009; figure 1). Moreover, drilling
of pearls may remove the evidence laboratories need to
determine their origin (Crowningshield, 1986a,b). Thus,
there is a need to improve the acquisition of X-ray images
of pearls—for example, through the use of multiple images
taken in different directions and employing almost real-
time micro-radiography. Even so, determination of the nat-
ural or cultured origin of a small number of pearls remains
difficult with radiography alone (see questionable cases in
Sturman, 2009). Recently, X-ray computed microtomogra-
phy has shown promise for pearl identification (Strack,
2006; Wehrmeister et al., 2008; Kawano, 2009; Krzemnicki
et al., 2009).
Developed in the 1960s, computed tomography (CT or
µ-CT for computed microtomography), allows the user to
investigate nondestructively the internal structure of an
object with high spatial resolution, providing applications
for biology/medicine, materials science, and geology (see
Ketcham and Carlson, 2001; Van Geet et al., 2001; Jacobs
and Cnudde, 2009). It works by iteratively taking radio-
graphic projections of a rotating sample (usually through
360°; figure 2). X-rays are attenuated by the sample as a
function of its thickness and the linear attenuation coeffi-
cient (also known as the absorption coefficient—in this
case, how easily the material can be penetrated by the X-
rays) of the material. Projections of the sample are typical-
ly recorded by a CMOS (complementary metal-oxide
semiconductor) flat-panel detector with an integrated scin-
tillator. These projections are used to reconstruct three-
dimensional (3D) models of the investigated object. The
two-dimensional (2D) slices are cuts of the 3D models in
different directions. Depending on the size of the studied
area, it is possible to attain resolutions down to the
micrometer scale. Resolution is generally given as the
See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 46, No. 2, pp. XX–XX.
S. Karampelas, J. Michel, M. Zheng-Cui, J.-O. Schwarz, F. Enzmann,
E. Fritsch, L. Leu, and M. S. Krzemnicki
X-ray computed microtomography reveals the
internal features of pearls with great detail. This
method is useful for identifying some of the nat-
ural or cultured pearls that are difficult to sepa-
rate using traditional X-radiography. The long
measurement time as well as the cost of the
instrumentation and associated accessories are
some of this method’s disadvantages.
NOTES & NEW TECHNIQUES
X-RAY COMPUTED MICROTOMOGRAPHY
APPLIED TO PEARLS: METHODOLOGY,
ADVANTAGES, AND LIMITATIONS
NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2010 3
length of one pixel, or expressed as a volume element
(voxel, a 3D pixel). To be resolved, features must be several
voxel in dimension in at least one direction.
In this study, all the results are gray scaled—the radio-
graphs as well as the 2D and 3D slices/models. In the
radiographic images, lighter colors indicate materials with
higher density (e.g., calcium carbonate) and darker colors
represent lower-density materials (e.g., organic matter or
voids). With longer measurements, the calcium carbonate
polymorphs can be separated (e.g., aragonite from calcite
and vaterite: see Wehrmeister et al., 2008; Soldati et al.,
2009).
MATERIALS AND METHODS
More than 50 samples known to be natural pearls and
beaded or non-beaded cultured pearls from various rep-
utable sources were imaged and compared using X-radiog-
raphy and X-ray µ-CT. This study includes the results for
16 of these samples, representing different pearl categories:
6 natural pearls, as well as 4 beaded and 6 non-beaded cul-
tured pearls (both freshwater and saltwater) from various
mollusks (see table 1). Five were drilled or half-drilled; two
were mounted in jewelry (see table 1).
Film X-radiography was done at the Gübelin Gem Lab,
following the standard techniques used in most gemologi-
cal laboratories (see, e.g., Akamatsu et al., 2001). X-rays
were generated by a Comet X-ray unit, and the samples
were immersed in a lead nitrate solution (used as scatter-
reducing fluid). Two or more radiographs were taken in
different directions for all samples. Each image required
about 20 minutes.
Microtomography measurements were performed at
the Institute of Geosciences of Mainz University, using a
ProCon X-Ray CT-Alpha instrument equipped with a
YXLON FXE 160.51 X-ray tube and a Hamamatsu flat
panel sensor detector (figure 3). Although this instrument
is capable of taking images that are 2048 ¥2048 pixels (50
m per pixel), all of the images we used were taken with a
setting of 1024 ¥1024 pixels (100 m per pixel); that is, four
pixels were merged as one during image recording. This
procedure allowed for a shorter measurement time and
smaller volumes of data, but it halved the given resolu-
tion. The instrumentation could measure objects as small
as 1 mm or as large as ~100 mm wide and 90 mm tall. The
highest resolution could be obtained by placing small
objects close to the X-ray source; this was done in some
cases to more closely examine interesting or questionable
structures. So-called region-of-interest (ROI) scans allow
imaging of larger objects or magnifications of a specific
part of an object. The drawbacks of such scans are typical-
ly an increase in artifacts and poorer image quality. The
sample chamber is 30 ¥30 ¥30 cm. Unlike radiography,
microtomography can only image one pearl (loose or, in
some cases, mounted) at a time.
A series of tests were run to define the ideal parameters
for the highest contrast between the different phases in
pearls. X-rays were generated with 100 kV accelerating
voltage and 110 µA target current. The beam was pre-fil-
Figure 1. Some of these white to yellowish brown
(“golden”) pearls are difficult to identify by classical
X-radiography, as they present questionable struc-
tures. A mixture of natural and cultured pearls (up to
~9.5 mm) are shown here. Photo by Evelyne Murer.
PC
X-ray
source
Cone beam
Sample on
rotary table
Flat-panel detector
2D projection
Data-reconstruction
image processing
2D slices
and 3D models
F
Figure 2. Components
of the µ-CT analytical
process are shown in
this schematic drawing.
4NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2010
tered by 1-mm-thick aluminum foil to reduce beam-hard-
ening effects. Projections were taken with an exposure
time of 500 milliseconds. Each measurement consisted of
800 projections (over a 360° rotation), averaging 10 images
for each position. Resolution was strongly dependent on
the size of the studied area, and ranged from 6.4 to 13.8
µm per voxel (see table 1).
Reconstruction of the raw data was done using Volex
software developed by the Fraunhofer Institute, Germany,
and image processing employed Amira software. Data
were output as 2D slices in the x-, y-, and z-directions, and
as 3D models. Each sample required about five hours for
analysis (including sample mounting, machine set-up,
measuring time, and data/image processing). All the calcu-
lations were carried out on PCs with >8 GB RAM. The
data generated for each pearl consumed >10 GB of comput-
er disk space.
NEED TO KNOW
• Computed microtomography can reveal the
internal structure of a pearl with micrometer-
scale resolution.
• The technique is particularly effective for identify-
ing non-beaded cultured pearls.
• Drawbacks include artifacts produced by sample
rotation and metal mountings, long measurement
time, large data files, and costly instrumentation.
TABLE 1. Characteristics and µ-CT resolution of the studied pearl samples.a
µ-CT
Sample no. Type Mollusk Size (mm) Shape Color Condition resolution
(µm)
SK-61 Natural SW Pteria spp. 8.7 ¥8.1 Drop Gray-black Drilled 11.0
(mounted)
SK-46 Natural SW Pinctada spp. 6.4 Round Light “cream” Drilled 7.0
Pp07 Natural SW Pteria spp. 10.1 ¥6.5 ¥3.9 Baroque Light “cream” Sawn 11.0
GGL03 Natural FW Unionida order 9.5–10.2 ¥7.0 Button Light gray Whole 10.8
GGL33 Natural FW Unionida order 6.9–7.4 ¥6.3 Button White Whole 8.0
GGL27 Natural FW Unionida order 6.0 Round White Whole 6.4
GGL17 Beaded SWCP P. margaritifera 9.7 Round Gray-black Whole 10.6
GGL18 Beaded SWCP P. margaritifera 10.4 ¥9.9 Drop Gray-black Whole 11.3
GGL19 Beaded SWCP P. maxima 12.7 ¥11.3 Button White Whole 13.8
GGL32 Beaded FWCP Hyriopsis spp. 6.9 Round Light gray Drilled 7.7
GGL22 Non-beaded SWCP P. margaritifera 10.6 ¥4.9 ¥2.8 Baroque Light gray Whole 11.0
SK-50 Non-beaded SWCP P. sterna 7.2 ¥5.4 ¥4.2 Baroque Gray-purple Whole 8.0
SK-51 Non-beaded SWCP P. sterna 7.2 ¥3.9 ¥3.5 Baroque Gray-purple Whole 8.0
SK-54 Non-beaded FWCP Hyriopsis spp. 10.0 ¥8.8 Drop Light gray-purple Half-drilled 11.0
SK-62 Non-beaded FWCP Hyriopsis spp. 11.0 ¥8.9 Drop Gray-purpl Half-drilled 10.0
(mounted)
GGL26 Non-beaded FWCP Hyriopsis spp. 6.3 ¥6.0 Near round Yellowish brown Whole 6.9
aAbbreviations: FW = freshwater, SW = saltwater, FWCP = freshwater cultured pearl, SWCP = saltwater cultured pearl.
Figure 3. For microtomography, we used the ProCon
X-Ray CT-Alpha instrument based at the Institute of
Geosciences of Mainz University. The outer dimen-
sions are 190 ¥100 ¥150 cm, and the sample cham-
ber is 30 ¥30 ¥30 cm. The total weight of the instru-
ment is 2.5 tons. Photos by J. Michel.
NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2010 5
RESULTS AND DISCUSSION
Selected results are shown in figures 4–8 (as well as in the
G&G Data Depository at gia.edu/gandg), which provide
photos of the samples, X-radiographs, 3D µ-CT models,
and 2D µ-CT slices in the most informative directions. For
the purpose of visualization, a portion of each 3D model
has been removed to show the internal structures. The
best structural visualization of the samples is revealed by
the 3D µ-CT images. In addition to producing superior
image quality, microtomography allows the user to scroll
through a pearl virtually by combining the single CT sec-
tions into a “movie,” enabling the dynamic recognition of
internal structures that are difficult to interpret when
observing single CT sections or radiographs (see G&G
Data Depository for this article and for Krzemnicki et al.,
2010).
All the beaded cultured pearls as well as all but two of
the non-beaded cultured pearls and natural pearls in this
study could be identified by radiography. The beads in the
BCPs (figure 4), the structures associated with the grafted
tissue in the NBCPs (figure 5), and the onion-like layers
with a black point in the center of the natural pearls (figure
6) typically were clearly seen in the radiographs. In some
cases, however, the µ-CT scans revealed additional charac-
teristics useful for pearl identification. In figure 7, for exam-
ple, it can be seen that a drill hole removed part of the
pearl’s central structures, and identification with tradition-
al radiography was uncertain. Although some growth struc-
tures appear on the radiograph, the µ-CT images reveal a
more detailed and three-dimensional view of the central
growth structures that enabled the identification of this
pearl as natural. Additional features such as cracks and
growth lines were also revealed in some of the µ-CT
images. These characteristics were about 10 µm thick (or
less), and were not observed with radiography.
The µ-CT technique does have some limitations.
Pearls that are mounted or that have a metal lining within
the drill hole may show artifacts, which can mask the
internal structures and thus make their identification diffi-
cult (figure 8). In radiographs, the metal mounting is less of
an obstacle. Also, µ-CT sections show reconstruction arti-
facts due to rotation. Although these artifacts can be
reduced with appropriate analytical parameters, generally
they are not completely removed. The artifacts are mani-
fested as perfectly centered fine circles in horizontal sec-
tions (i.e., transaxial sections), and as a blurry rotation axis
in the center of the reconstructed image in vertical sec-
tions (i.e., sagittal and coronal sections, which are oriented
90° to one another), as illustrated in Data Depository
items 1–6. Care must be taken so the fine circles in the
transaxial sections are not misinterpreted by an inexperi-
enced observer as (natural) onion-like growth structures.
More structures in natural and cultured pearls observed
with µ-CT are well illustrated by Krzemnicki et al. (2010), in
the G&G Data Depository, and at www.gubelingemlab.ch.
Figure 4. In this (a) white button-shaped beaded
saltwater cultured pearl from P. maxima (sample
GGL19), the bead is visible in the radiograph (b) as
well as in the 3D (c) and 2D (d) µ-CT images; how-
ever, the boundary between the nacre and the bead
is sharper in the µ-CT images, which also show
organic matter surrounding the bead. (See also
Depository item 1.)
Figure 5. In this (a) gray-purple baroque-shaped non-
beaded saltwater cultured pearl from Pteria sterna
(“keshi”; sample SK-51), tissue-related structures are
visible in the radiograph (b) and µ-CT images (c, d).
Characteristically, these structures follow the shape of
the pearl. Some finer-scale structures are seen in the µ-
CT images. (See also Depository item 2.)
6NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2010
Figure 6. In this (a) light gray button-shaped freshwa-
ter natural pearl from the Unionida order (sample
GGL03), typical onion-like structures with a black
point in the center are visible in the radiograph (b) as
well as in the µ-CT images (c, d), but are sharper in
the latter. The µ-CT images also reveal fissures sur-
rounded by a denser (white-appearing) material,
which are barely visible in the radiograph. (See also
Depository item 3.)
Figure 7. In this (a) drilled, light “cream,” round salt-
water natural pearl from Pinctada spp. (sample SK-46),
concentric growth structures are observed in the radio-
graph (b), but the drilling has partially removed the
structures in the center of the pearl and identification
with the radiograph alone is inconclusive. In the µ-CT
images (c, d), however, some remnants of the central
growth structures are visible, revealing the pearl’s nat-
ural origin. (See also Depository item 5.)
Figure 8. In this (a) mounted, half-drilled, gray-purple, drop-shaped non-beaded freshwater cultured
pearl from Hyriopsis spp. (sample SK-62), characteristic structures of a cultured pearl are observed in
the radiograph (b) as well as in the 3D µ-CT image (c). However, in the µ-CT image the metal partially
masks the internal structure of the pearl. (See also Depository item 7.)
NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY SUMMER 2010 7
CONCLUSION
Although most cultured and natural pearls can be reliably
separated with radiographs only, their biomineralization is
better visualized with µ-CT (unless they are mounted in
metal). In fact, some non-beaded cultured pearls require
high-resolution 3D imaging for a correct identification; in
such cases, µ-CT can be quite helpful.
The main advantage of tomography is that it gives
high-resolution three-dimensional information, whereas
radiography condenses the 3D structures onto a flat film as
a two-dimensional image. This becomes evident when
observing fissures in pearls. Their position within the 3D
volume of a pearl is sometimes difficult to interpret in
radiographs, even when they are taken in different direc-
tions. With µ-CT, the tissue-related structures and the fis-
sures are better revealed, so it is easier to make a correct
identification. However, the technique is mainly useful for
pearls that do not have metal mountings, it requires a long
measurement time, and it consumes a huge amount of
data storage space. In addition, µ-CT instrumentation is
still costly—about US$550,000 for the instrument and
accessories—and the technique requires scientifically
trained staff for analysis and interpretation. Note, though,
that a new generation of instruments using X-rays are
entering the market, which could be used for digital radio-
graphy as well as µ-CT, at the same or even lower prices.
Additional µ-CT studies of problematic pearls (e.g., the
non-beaded types described by Sturman, 2009) are expect-
ed to reveal more of the strengths and limitations of this
emerging method. Studies at higher resolution, magnifying
a specific region of the sample (such as with synchrotron
µ-CT), may reveal some very small details of pearl struc-
ture that are useful for their identification. Micro-CT anal-
ysis may also prove helpful for identifying organic gem
materials protected by CITES, such as corals and ivory.
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ABOUT THE AUTHORS
Dr. Karampelas is a research scientist, and Mrs. Mingling
Zheng-Cui is analyst, at the Gübelin Gem Lab, Lucerne,
Switzerland. Mr. Michel, Dr. Schwarz, and Dr. Enzmann are
researchers, and Mr. Leon Leu is an undergraduate student,
in the Institute of Geosciences at Johannes Gutenberg
University of Mainz, Germany. Dr. Fritsch is professor of
physics at the University of Nantes, Institut des Matériaux
Jean Rouxel (IMN) - CNRS, Team 6205, France. Dr.
Krzemnicki is director of the SSEF Swiss Gemmological
Institute, Basel.
ACKNOWLEDGMENTS
The authors thank Thomas Hainschwang (Gemlab, Balzers,
Principality of Liechtenstein), Edigem Ltd. (Lucerne,
Switzerland), Centre de Recherche Gemmologique
(University of Nantes, France), and Perlas del Mar de Cortez
(Guaymas, Mexico) for providing some of the study samples,
as well as Alessandra Spingardi (Gübelin Gem Lab) for the
photos of the samples.
