Fall 2000 Gems & Gemology - Gemfrance

Figure 2. Composed of tiny angular flakes of iridescent
ammonite shell encased in a plastic that is
sandwiched between a glass cap and a black opaque
backing material, these two assembled cabochons
make attractive use of otherwise unusable fragments.
The oval measures 9.90 × 7.58 × 3.59 mm,
and the round piece measures 7.01–7.09 × 3.08 mm.
Photo by John I. Koivula.
plastic that encased small angular fragments of fossilized
iridescent ammonite shell, and a backing layer of opaque
black material. The central plastic layer and the black
backing melted when a thermal reaction tester was
applied.
The assemblages showed no fluorescence to longwave
UV radiation, but they did fluoresce a strong,
chalky, slightly bluish white to short-wave UV. When we
examined the samples without magnification, as well as
with a microscope set up for UV examination, it was our
general impression that the fluorescence came from the
Figure 3. With magnification, the jagged edges of the
individual ammonite shell fragments are clearly visible.
Notice the small gas bubble trapped in the plastic
layer at the left edge of the assembled cabochon.
Photomicrograph by John I. Koivula; magnified 15×.
glass dome, and its glow caused the center layer to appear
fluorescent as well. Disassembly of one of the cabochons
would be required, however, to confirm which components
were actually responsible for the reaction.
On a Gem Trade Laboratory identification report,
this material would be identified as: “Triplet, consisting
of a glass top and mosaic inlay of natural fossilized
ammonite shell fragments with a thick black backing.”
Gemmy anhydrite from Iran. Faceted anhydrite is a very
rare collector’s stone, with only a few known localities in
the world. It is difficult to cut, as it has a low hardness (3.5)
and perfect cleavage in one direction. A new find of facetable
anhydrite (figure 4) comes from Iran’s Hormoz and
Qeshum Islands in the Persian Gulf. A few dozen stones
were recovered on the surface of a typical salt dome by a
Czech speleological expedition that was exploring very
unusual large caves formed from massive halite.
The anhydrites occurred as loose crystals up to about
8 cm in length, but usually only small portions were
transparent. One specimen with anhydrite growing on
quartz crystals was found. The surfaces of some anhydrite
crystals were “parqueted” (similar to heliodor crystals
from Ukraine) due to natural etching. Twinned crystals
were rare. The anhydrite crystals examined were colorless,
light violet, or (very rarely) pink (see, e.g., figure 4
inset). Refractive indices measured on a polished oriented
crystal were n a = 1.570, n b = 1.576, and n g = 1.616,
with a high birefringence of 0.046. Specific gravity (measured
hydrostatically) was 2.95-2.96. The crystals showed
no lines in the visible spectrum with a hand spectroscope,
and no fluorescence to UV radiation.
Typical inclusions seen with a microscope were multiphase
negative crystals as well as at least three types
of colorless crystals. One type was identified as quartz
on the basis of its shape (figure 5) and bright interference
colors (seen even in plane-polarized light). Groups of
unidentified rounded anisotropic crystals were found in
another cut stone. Isotropic included cubes were probably
halite. Some stones also contained small black
crystals that are probably hematite. Almost all the cut
anhydrites contained parallel mirror-like cleavage planes.
The approximately 20 faceted anhydrites examined
ranged from 1 to 5 ct, although one 22.74 ct stone has
already been faceted (again, see figure 4) and even larger
rough exists.
The area also has produced facetable colorless and
purple fluorite, yellow apatite (very similar to the material
from Durango, Mexico), and colorless danburite
(although this last material would produce only small
cut stones). Other minerals include bipyramidal
hematite crystals, dolomite twins, halite, pyrite, and
augite. The quartz crystals occasionally found were
milky due to abundant, very fine curved fibers (which
were not identified).
Jaroslav Hyrsl (Hyrsl@kuryr.cz)
Kolin, Czech Republic
262 Gem News GEMS & GEMOLOGY Fall 2000