The beauty of gemstones is mostly dependent on their color, diaphaneity,
brilliancy, luster, and fire. Any one or a combination of these
properties render stones desirable as gems.
Color is very important in many gemstones. The color of transparent
varieties should be distinct enough to be pleasing to the eye, yet not
so dark as to appear black or opaque. It is generally more desirable
that the gemstone be of even color and not appear “patchy” or
“streaked.” However, some opaque or translucent stones such as agate owe
their popularity chiefly to the variety of colors and designs within a
single piece. Some transparent gemstones exhibit different colors when
viewed in different directions. For example, some fine blood-red rubies
appear brownish when viewed in a particular direction. The gemstone
should be cut so that its finest color is most prominently displayed.
This ability of some gemstones to exhibit different colors when viewed
in different directions is called pleochroism.
Diaphaneity is the relative ability of stones to transmit light.
Diaphaneity is described by terms such as transparent, translucent, and
opaque. Transparency is highly desirable in stones such as diamond that
are commonly facet-cut to reflect light. The gemstone should be water
clear and free from inclusions and cracks so that it transmits light
freely, but there are stones that do not exhibit this property that are
prized as gemstones. For example, turquoise may appear to be completely
opaque and not transmit any light, but it is sought for its fine blue
color.
The brilliancy of gemstones is largely dependent on their index of
refraction. The index of refraction is a measure of the ability of a cut
gemstone to “bend” light rays and reflect them from the bottom facets
back through the top of the stone. Of course, brilliancy is not noted in
opaque or faintly translucent stones. The index of refraction of
gemstones is expressed numerically. Air is the reference medium and is
assigned an index of refraction of 1.00. Other substances are assigned
values relative to that of air, for example, water, 1.33; topaz, 1.62;
diamond, 2.42. The higher the index of refraction, the more brilliant
will be the gemstone if it is properly cut and polished.
Luster is the appearance of the mineral on a fresh surface in reflected
light; it is divided into two major categories, metallic and
non-metallic. Most gemstones have non-metallic luster and are described
by terms such as vitreous or glassy, resinous, waxy, greasy, and pearly.
The fire, or ability of gemstones to show flashes of different colors of
light, is dependent upon a property called dispersion. The amount of
dispersion is the extent to which the gemstone is able to separate
ordinary white light into its component colors. The dispersion of
gemstones can also be expressed numerically but for purposes of this
publication will be referred to as low, moderate, or high. Diamond is a
common gemstone that has high dispersion.
A gemstone’s durability is primarily dependent upon its hardness. The
Mohs scale of hardness, given below, is most commonly used for gemstones
and other minerals.
_Mohs Scale of Hardness_
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond
On this scale, the higher numbers are the harder minerals. Mohs is a
relative, not an absolute scale. Therefore, it should not be assumed
that diamond is ten times harder than talc because actually diamond is
very many tens of times harder than talc. However, a particular mineral
is harder than any other mineral with a lesser number, and the scale is
very convenient to use. Gemstones mounted in rings should have a
hardness of at least seven on the Mohs scale, or the stones may become
scuffed and scratched after a relatively short period of wear. Gemstones
mounted in pins and brooches can be of softer material as they are not
usually subjected to abrasion and rough treatment.
The tendency of some minerals to split with relative ease in particular
directions along planes is called cleavage. Cleavage is also a factor
determining the durability of gemstones. Some gemstones do not exhibit
this tendency at all, whereas others cleave in several directions. The
number of cleavages is always the same in any one mineral, and the
direction of cleavages is constant in relation to the crystal structure
of any one mineral or gemstone. It is apparent that of stones having the
same hardness, the ones lacking cleavage or having the lesser number of
good cleavage directions are the most durable.
Some stones, such as jade and agate, owe their durability to their
compact fibrous structure, which makes them very tough and durable even
though they are not especially hard.
Several other properties of gemstones, although not always contributing
to the beauty or desirability of gemstones, are useful in identifying
uncut specimens.
Streak is the color of the mineral when finely powdered or, for softer
minerals, the color obtained by rubbing the mineral against a piece of
unglazed porcelain or tile. The color of a mineral’s streak is commonly
different from the unpowdered specimen.
Fracture is the kind of surface obtained when the mineral is broken in a
direction that is not a cleavage direction. Fracture surfaces are
described by such terms as conchoidal (like the fracture of glass),
subconchoidal, splintery, even, and uneven.
Tenacity is the resistance of a mineral to breakage. Brittle minerals
break relatively easily on impact. Malleable minerals, such as gold, may
be flattened under a hammer into very thin sheets without breaking.
Sectile minerals may be cut with a knife without powdering. Most
gemstones, even diamond, are brittle.
It is only natural to value most those gemstones that are not common or
easy to obtain. Emerald owes its longstanding popularity to its fine
green color, but tourmaline is sometimes found in colors that very
closely approach that of emerald and yet sells for considerably less
because it is so much more common.
Rarity is not the only factor affecting the value of gemstones. Freedom
from internal imperfections, quality of cutting, color, and size must
also be considered in cut and polished gemstones. Internal
imperfections, such as inclusions and cracks, detract from the
appearance of gemstones and interfere with the passage of light between
the facets; consequently, gemstones containing these imperfections are
not valued as highly as those without them. Poor cutting or polishing
detract from the beauty and thus from the value of gemstones. Unpopular
or poor color commonly causes gemstones to be less valuable. Rich green
emeralds are exceedingly prized, whereas very pale green emeralds are
relatively inexpensive. Diamonds that have the least hint of yellow are
never valued as highly as pure colorless, pink, or blue stones. Few
persons find the yellowish color attractive, unless it is a vivid canary
yellow.
Size is important in determining the value of gemstones but not as
important as perfection. A badly flawed gemstone of large size may be
worth only a slight fraction of the value of a smaller perfect one.
Gemstone size is usually measured in carats, a unit of weight, although
millimeter size is sometimes used. Five carats is equal to 1 gram and
approximately 28⅓ grams is equal to 1 ounce avoirdupois. One
one-hundredth (0.01) of a carat is called a point, and this term is
often used, especially pertaining to very small gemstones.
The term used to compare the relative weights of minerals and gemstones
is specific gravity, which is expressed numerically in relation to
water. Water is assigned the value of 1.00. Therefore, at a given
temperature a gemstone having a specific gravity of 2.00 is twice as
heavy as an equal volume of water. A 1-carat sapphire (specific gravity
about 4.00) will be smaller than a 1-carat amethyst (specific gravity
about 2.65) because the heavier material will occupy less volume to have
the same weight.
A summary of properties helpful in identification of common Texas gem
minerals is given in Table 1.
Comparatively recently in the history of gemstones, man has succeeded in
the production of synthetic gems that have properties closely
approaching those of many natural gemstones. To the untrained eye some
synthetic gems may appear identical to natural stones, but synthetic
gems can be detected with little difficulty by a properly equipped
expert. Although most synthetic gems are inexpensive, their manufacture
has not adversely affected the value of natural gemstones but instead
has increased the demand for fine natural gems.