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Glossary
Absorption
refers to the spectrum of light that is absorbed in the stone, which
gives the stone its color. An amethyst, for example, absorbs the various colors
of the spectrum with the exception of purple, which is dispersed, causing it to
be appear the purple color we recognize.
Asterism
is a dense, yet minute, fibrous inclusion that causes the mineral to
reflect a star like formation. Usually an asterism is four, six or twelve rays
and is almost always limited to a cabochon type cut. Star sapphires or star rubies
are perhaps the best known stones featuring asterisms.
Birefringence
or double refraction is the phenomenon exhibited on all minerals that are isometric
and not opaque. A light ray enters the crystal and splits up into two separate rays,
making anything observed through the crystal appear as double. The double refraction
on most minerals is so weak that it cannot be observed without a refractometer. The
Iceland Spar variety of Calcite is an exception; it is strongly seen unaided. The
double refraction is different in every mineral, and thus can be used to identify gems.
Chatoyancy
is the property whereby a stone will display a concentrated narrow
band of light across the surface of the stone such as in tiger-eye. Chatoyancy
is similar to asterism and is also found almost exclusively in cabochons of a
fibrous structure.
Dichroism
is the tendency to show two different colors from different angles
when looking at the gemstone. Tourmaline is a common example of a
dichroic stone, appearing green or pink depending on the angle it
is viewed from.
Dispersion
refers to the color that is allowed to disperse, or pass through a
stone. A high quality diamond disperses virtually all colors, and is therefore
colorless to our eyes.
Double Refraction
see Birefringence
Fluorescence
Property of certain minerals to display a multicolored effect when lluminated with
ultraviolet light.
Hardness
The resistance of an object to scrapes and scratching. The harder it is, the greater its
resistance.
Inclusion
Materials that are locked inside a mineral as it is forming. They may be be gaseous, liquid
or solid.
Isomorphous
Minerals that have the same molecular arraingement, or crystal form, but that are composed
of different elements in the same ratio, such as is found in the garnet group. Almandine
Garnet (Fe3Al2(SiO4)3), and Pyrope Garnet (Mg
3Al2(SiO4)3) both contain 3 silicates
(SiO4) and two Aluminum atoms. they differ in that one contains Iron, the other
an equal amount of Magnesium
Luster
is the optical property that describes the appearance of the gem in terms
of the reflected light. The stone may be described as metallic, adamantine,
vitreous, resinous, greasy, silky, pearly, or dull. Luster is most importantly
used in referring to the uncut gem.
Play of color
describes an effect where flashes of brilliant color can be seen in
the stone, yet they change in both color and size as the stone is moved. Light
reflecting from tiny cracks and imperfections inside the stone causes this
phenomenon. Play of color is not to be confused with fire as seen in other gems,
which is the reflection of light from off of the interior side of a facet. Opals
ore best known for showing a play of color.
Pleochroism
is the tendency in a mineral to exhibit two or more separate colors
when viewed at different angles. Pleochroism and dichroism are
synonymous, except dichroism refers only to two colors, but
pleochroism can be more than two.
Refraction
is the bending of light within the stone. It is due to different
spectrums of light traveling at different speeds through the stone. The
greater the refractive index, the greater the dispersion of color, and therefore
the greater brilliance or 'sparkle'. Diamonds, known for their 'fire', have a
refractive index or R.I. of 2.42 while the cool green emerald comes in closer to
1.58 with little fire. Refractive index can be measured with a refractometer.
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