Soft 1. Talc
^ 2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
v 9. Corundum
Hard 10. Diamond
It is difficult to list these characteristics in the order of
importance, but _hardness_ would rank high. Hardness of a gem is best
defined as its resistance to abrasion or scratching. Most commonly used
for comparison is the Mohs scale, which consists of selected common
minerals arranged in the order of increasing hardness. On this scale,
topaz is rated as 8 in hardness, ruby as 9, and diamond, the hardest
known substance, as 10. Any gem with a hardness less than that of
quartz, number 7 in the scale, is unlikely to be sufficiently
scratch-resistant for use as a gem. A less precise scale, using common
objects for comparison, might include the fingernail with a hardness up
to 2½, a copper coin up to 3, a knife blade to 5½, a piece of window
glass at about 5½, and a steel file between 6 and 7, depending on the
type of steel. By this scale, any stone that remains unmarred after
being scraped by a piece of window glass will have a hardness greater
than 5½. The more important gemstones—which include diamond, ruby,
sapphire, and emerald—all have a hardness much greater than 5½.
The size of a gemstone usually is indicated by its _weight_ in carats.
The expression “a 10-carat stone” has meaning—if somewhat inexact—even
to the nonexpert. Specifically, a carat is one-fifth of a gram, which is
a unit of weight in the metric system small enough so that approximately
28 grams make an ounce. A 140-carat gemstone, then, weighs about an
ounce.
Another distinguishing characteristic of a gemstone is its specific
gravity, which is an expression of the relationship between the stone’s
own weight and the weight of an equal volume of water. We are aware of a
difference in weight when we compare lead and wood, yet it would not
always be correct to say that lead weighs more than wood, for a large
piece of wood can weigh more than a small piece of lead. Only by
comparing equal volumes of these materials can the extent of the weight
difference be clear and unmistakable. Diamond is 3½ times heavier than
the same volume of water, so its specific gravity is 3.5. Since each
species of gem has its own specific gravity, which can be determined
without harming the stone, this standard of comparison is a valuable aid
in identifying gems. Several techniques have been devised for
determining specific gravity, and most of them make use of some kind of
weighing device or balance.
Among the most striking and useful of the distinguishing characteristics
of gemstones are those that involve the effects on light.
An important effect of a gem on light is the production of color, upon
which many gems depend for their beauty. Some gem materials, such as
lapis lazuli, have little to offer except color. Many gemstones vary
widely in color, owing to the presence of varying but extremely small
amounts of impurities. Thus, the gemstone beryl may occur as blue-green
(aquamarine), as pink (morganite), as rich green (emerald), as yellow
(golden beryl), or even colorless (goshenite).
[Illustration: Sketch of a simple balance used to determine specific
gravity of a gemstone. The operator places the gemstone in the upper
pan (A), moves the weight (B) along the beam (C) until it balances
perfectly, and notes the number at the weight’s position. He then
transfers the gemstone to the lower pan (D), which is completely
immersed in water, and moves the weight along the beam to restore
balance. He notes the scale number at the new position and
determines the specific gravity simply by dividing the first number
by the difference between the two numbers. If the gemstone is large,
the operator can use heavier sliding weights. (E).]
Gemstones such as beryl and sapphire that depend on impurities for their
color are said to be _allochromatic_; others, such as peridot and
garnet, which are highly colored even when pure, are said to be
_idiochromatic_. The color of a gem is further described according to
its _hue_, _tint_, and _intensity_. Hue refers to the kind of color,
such as red, yellow, green, etc.; tint refers to the lightness or
darkness of the hue; and intensity refers to vividness or dullness.
Throughout history, the most popular colored stones have been those with
hues of red, green, or blue of dark tint and high intensity.
[Illustration: A 43-carat albite from Burma (at left), 76-carat
tourmaline from Brazil, and 30-carat wernerite from Burma exhibit a
strong cat’s-eye effect because of reflection from inclusions in
parallel arrangement within the stones. (Actual size.)]
[Illustration: Asterism (star effect) is caused by parallel
inclusions arranged in several directions related to the crystal
structure of the gemstone. Two rays in the 175-carat, 6-rayed star
garnet from Idaho (at left in photo) are weaker than the other four
because of fewer inclusions in that direction. The 23-carat star
orthoclase from Ceylon shows brightly all of its four possible rays.
(Actual size.)]
The effect of a gem on light may be more than the production of color.
Several of the so-called phenomenal stones are prized for other effects.
Holes, bubbles, and foreign particles, when properly aligned in parallel
groupings, can produce interesting light effects. The play of colors of
opal and labradorite, the _chatoyancy_ or silky sheen of tiger’s-eye and
cat’s-eye, the _opalescence_ or pearly reflections of opal and
moonstone, and the _asterism_ or star effect of rubies and sapphires are
caused by the reaction of light to minute _inclusions_ or imperfections
in the gemstone.
When light passes into or through a gemstone with little or no
interruption, the stone is said to be transparent, as opposed to a stone
through which light passes with greater difficulty, and which is said to
be either translucent or opaque, depending on the degree of light
interruption.
[Illustration: Rays of light passing into a gemstone are refracted
(bent) in varying amounts depending on the gem species and also on
the angle at which the light strikes the stone. The light rays are
reflected back toward the top of the stone by internal faces
(facets), and they are refracted again as they leave.]
[Illustration: How a gem refractometer, a simple device to operate,
is used to measure quickly the refractive index of a cut gemstone. A
light beam passing through the opening (A) is reflected from the
table of a gemstone (G) through a lens system (L) and, by prism (P),
into the eye of the observer (E). The maximum angle of reflection
(N), which depends on the refractive index of the gemstone, controls
the angle at which the beam comes through the eyepiece (EP). The
refractive index is read directly from a scale in the eyepiece.]
The action of a gemstone upon the light which strikes its surface and is
either reflected or passed through it sometimes results in highly
desirable effects that enhance its beauty and aid in its identification.
Light passing into a stone is bent from its path, and the amount of
bending (_refraction_) depends upon the species of the gemstone. When
the degree of bending can be measured, the gem species can be
identified, since very few species of gemstones bend light to exactly
the same degree. An instrument called a gem refractometer is used to
determine the degree to which cut stones refract, or bend, light. The
measurement obtained is the _refractive index_ of the gemstone.
Many gemstones can split a beam of light and bend one part more than the
other, thus producing _double refraction_, or two different measurements
of refractive index.
[Illustration: When a ray of ordinary white light enters some
gemstones it is dispersed (split up) into rays of the separate
colors of which it is composed. These rays are reflected inside the
gem and are further separated by additional refraction as they leave
the gemstone. This dispersion accounts for the colored flashes of
light, or fire, for which diamond is highly prized.]
Gems have the ability to separate “white light” (the mixture of all
colors) into its various colors, producing flashes of red, yellow,
green, and other colors. Separation occurs because the various colors,
or wavelengths composing white light passing through the gem, are each
bent or refracted a different amount. Red is bent least, followed in
order by orange, yellow, green, blue, and violet, which is bent most.
This characteristic of being able to produce flashes of color, as seen
prominently in diamond, is known as _dispersion_ or _fire_. Quartz and
glass have low dispersion, and hence they make poor diamond substitutes.
Some of the newer synthetic gemstones, such as titania, have extremely
high dispersion, with resulting fire. Zircon, a natural gemstone of
suitable hardness, exhibits high dispersion and is a commonly used
substitute for diamond.