_The Techniques of Gems_
I have been using the terminology of the field of gems and jewelry,
taking it for granted that the meanings would be understood. Perhaps it
is time to make these terms more precise.
_Definitions_
A jewel, or a piece of jewelry, is a costly ornament, especially of gold,
platinum, or precious stones; or of stones set in one of these metals.
A precious stone is one highly prized for human adornment. Its value
is measured mainly by its beauty, its rarity, and its durability. The
precious stones are, by general understanding, limited to the diamond,
the ruby, the emerald, and the sapphire. The pearl, though strictly not
a stone and far less durable, is nevertheless, because of its beauty and
the rarity of superb specimens, included among the precious gems.
A gem is a precious stone of rare quality, especially when cut and
polished. All other stones used in jewelry are semiprecious.
_Light on the Stones_
Stones may be characterized according to their response to light.
Lustrous stones are those which catch the light brilliantly and glow
almost as though with an inner flame. The cutting of the gem may aid
in this effect, as with the diamond. Vitreous stones are of the glassy
type, not lustrous. These may be transparent, permitting one to see
objects clearly through the stone, like a fine crystal; or they may be
translucent, permitting one to see light and shadow but not distinct
objects through the stone. When light falls upon such translucent stones
as moss agate, moonstone and agate, there is a soft glow.
Or stones may be opaque, permitting no passage of light, like the
turquoise. Because of their crystalline structure, even the opaque
stones, however, may respond glowingly to light. Sometimes when the
stone was formed, tiny cracks or bubbles stayed between the crystals. As
the stone is moved, these cracks cause a play of prismatic colors which
seems almost the sparking of an inner fire. To the names of such stones
the term _fire_ is prefixed; they are extremely rare and beautiful. The
Empress Josephine had a fire opal so remarkably aflame that she called it
“The Burning of Troy.”
_Star Gems_
Another variation from regularity, which can scarcely be called an
imperfection or a flaw, enhances the beauty and the value of a precious
stone. A certain break or tiny space in the crystalline structure may
produce a radiation of three lines crossing at a single point, giving the
effect of a six-pointed star. The star ruby and the star sapphire are
among the most highly prized of all gems.
_The Pearl_
The pearl has been described as “a disease of the oyster.” A tiny foreign
object, such as a grain of sand or a chip off the inside of the shell
(this inside is called nacre or mother-of-pearl) becomes imbedded in the
oyster itself; it is, of course, an irritant. Drawing upon its natural
resources but unable to expel the foreigner, the oyster protects itself
by isolating the intruder, building around the speck a thin layer of an
iridescent fluid, similar to that which lines the oyster shell. This
fluid hardens, layer after layer. Given proper time—about four or five
years—and the proper species of oyster—not the kind commonly used for
food—and a pearl is born.
A cultured pearl differs from an imitation pearl much as a synthetic
differs from a paste stone. A cultured pearl is naturally developed by an
oyster which has been artificially inseminated. Man starts the process,
the oyster carries it through. About 1920 an ingenious Japanese inserted
a tiny bead of mother-of-pearl into an oyster; the result was the first
cultured pearl. Since the oyster is first captured, then inoculated, then
released under controlled conditions, the processes of production can be
kept less haphazard, the time speeded and the quantity increased. As with
synthetic gems, however, there are tiny indications, in structure, in
lustrousness, by which the cultured may be distinguished from the native
pearl.
_Cutting the Stones_
I have mentioned that the cutting may help to bring out the brilliance
of a stone. There are two main types of cutting: the cabochon, used from
earliest times; and the faceting, used increasingly over the past four
centuries. Each is still valued for particular stones and purposes.
_Cabochon_
A stone cabochon cut is cut in a smooth upward (convex) curve, like
the arc of a circle or an ellipse. Most frequent is the medium cut,
a smooth oval with the under surface flat. The steep cut produces a
dome-like effect, as of a small haystack or high mound. In the hollow
cut, the upper surface is convex and the lower surface is concave, the
effect being that of a small bar curving upward. The fourth commonly
used cabochon cut is the double cut, the upper surface curving up and
the lower surface curving down, like a tiny elongated football. Which of
these cuts is used depends partly upon the jewel for which the stone is
intended, but mainly upon the original shape and coloring of the stone.
_Facets_
Transparent and translucent stones which seem to have radiance are
usually made more beautiful by faceting. A facet is a small, smooth face
or plane surface; a number of these are cut upon a gem.
In most facet-cut gems five regions can be distinguished. The table—the
top of the stone—is usually flat, though it may be slightly domed; it is
usually the largest of the facets, though the size may vary according
to the stone and the type of cut. The bezel is the slope from the top,
consisting of slanted, smooth faces that may proceed in various planes
or by ninety-degree “steps” down to the girdle. The girdle is the widest
part, the “equator,” of the cut stone; it is here that the setting is
usually attached. The pavilion is the part that slants down from the
girdle to the culet, which is the bottom point of the stone. Sometimes
the stone is slightly truncate; that is, it is cut to a small flat
surface, instead of a point, at the culet. More generally, the part of
the stone above the girdle is the crown; the part below the base.
_Types of Faceting_
There are many patterns of faceting. Six are fairly common.
1. Brilliant. This is used especially for large diamonds, which are then
often called brilliants. The gem is cut as though two pyramids with
sixteen-sided bases were placed base against base, the points at opposite
ends. The upper point is truncated, to form the table. Brilliant-cut gems
usually have 58 facets, 33 above the girdle, 25 below. For the sake of
the superb light effects achieved by this cut, there is often sacrificed
a considerable portion of the original stone.
2. Rose. This may be used for smaller diamonds and other gems. The rose
cut is circular, with the table slightly domed. It is flat underneath.
The part above the girdle is usually cut into 24 equal facets.
3. Square. This cut, as its name indicates, provides a square table. The
facets are cut parallel to the girdle, both above it and below. Since
they will thus seem to be proceeding downward in a succession of steps,
this is also called the step cut.
4. Emerald. The emerald cut may have either a square or an oblong table.
The corners, instead of being pointed and at right angles as in the
square cut, are cut off and faceted. As the name implies, this is a
frequent cut for the emerald, but the topaz, amethyst, aquamarine, and
other stones—even the diamond—may be square cut or emerald cut.
5. Marquise. The marquise cut is somewhat like an oval, but pointed at
the ends: boat-shaped. It is sometimes called navette.
6. Pear-shape. This very fancy shape is cut like a marquise but with one
side rounded out, giving a tear or drop-like appearance. It lends itself
very well to free-hanging parts on necklaces and earclips. This cut is
growing in popularity for an engagement ring.
There are many other possibilities of special faceting and fancy cuts.
Stones may be cut in the shape of a kite, a keystone, a lozenge, a
triangle, a half-moon or other figure. Popular among special shapes is
the baguette, “little stick,” in which the stone is cut to resemble a
small rod.
Increasingly in recent years, especially as a sentimental souvenir
and even more in the new engagement rings, diamonds are being cut
heart-shaped. This is a difficult and a costly pattern to produce since
not every diamond lends itself to be cut into heart shape.
Facet cuts have come to be far more frequent than cabochon. Cabochon,
usually in a medium cut, is still used for star rubies and star
sapphires, as its smooth surface most lavishly displays the radiance
of the star. Also, the moving band of light in the cat’s-eye and the
reflection in the moonstone are at their best in cabochon. When the color
in a ruby, garnet, or sapphire is beautifully deep, the curve of the
cabochon takes fullest advantage of that depth and richness. Cabochon cut
is also used for most opaque stones, as the opal, the turquoise, and the
jade. The baguette cut is most often used around a ring, or as a frame
for larger stones. Each cut has its separate beauty, and is designed to
bring out the richest qualities of its gem.
_Hardness of the Stones_
One reason for the pre-eminence of the diamond is its indestructibility.
It is by far the hardest of all stones. Setting the standard of the
diamond at ten, a table has been made of descending hardness. The whole
numbers on this scale are marked as follows:
10 diamond
9 corundum
8 topaz
7 quartz
6 feldspar
5 apatite
4 fluorspar
3 calcite
2 gypsum
1 talc
It is at once obvious that few of these are precious, or even
semi-precious, stones. What must be noted is that this list is not a
proportionate scale; that is, it indicates order, but by no means any
specific degree of hardness. The difference in hardness between the
diamond and its neighbor, corundum, is greater than that between corundum
and talc. The best that can be said is that, as they are arranged, each
one can scratch all those listed below it.
Thus there is no other stone that can scratch a diamond. The old saying
“diamond cut diamond” means that two champions are evenly matched,
and diamonds can be cut and polished only in this fashion. A wheel of
corundum or other substance is coated with diamond dust; when this is
applied to a diamond stone, an equal process of attrition takes place;
diamond dust is worn off both the wheel and the stone. This dust, of
course, may be used for further cutting and polishing to make the
finished stone.
The cutting referred to here is the shaping of the facets and the
surfaces of the stone; for most crystalline formations, however hard,
are brittle; that is, they may be split or cleaved along the lines of
the crystal edge. This accounts for both the possibility and the danger
of cleaving the raw diamond. Formed under tremendous pressure beneath
the surface of the earth, a diamond may be distorted in its growth;
it may be an unshapely and often a fairly large stone, which must be
cleft to a proper shape and size for setting. This cleaving, effected
by a single hammer tap, is made only after minute examination and
re-examination—sometimes a year’s pondering—by an anxious expert. After
this cleavage, a diamond will be much smaller than when it was mined,
but it counterbalances the loss of size by the greater brilliance and
beauty the new shape discloses. The Kohinoor diamond was over 700 carats
when it was found; when cut it was no more than 186⅙ carats; and since
then it was recut, as a brilliant, to its present weight of 106⅙ carats.
Sculpture has been defined as the process of removing the excess from the
marble statue already within the block; how much more this is true of the
precious stone, which the lapidary releases from its dull confinement!
Diamond dust, black diamonds, and hard metals may be used to shape,
engrave and polish the other stones. A list of some of the stones
frequently used in ornaments and jewels would rank them, for hardness, in
the following order:
diamond 10
sapphire 9
ruby 8.8
chrysoberyl 8.5
spinel 8
topaz 8
aquamarine 8
emerald 7.8
zircon 7.8
tourmaline 7.5
amethyst 7
bloodstone 7
chalcedony 7
onyx 7
jade 6.5
peridot 6.3
moonstone 6.3
turquoise 6
opal 6
lapis lazuli 5.2
pearl 4
malachite 3.5
coral 3.5
amber 2.5
jet 2.5
_Qualities of a Stone_
The qualities that determine the value of a stone are difficult to
specify. Hardness, size, weight and shape are obvious elements. Lustre
and the powers of reflecting and refracting light clearly contribute
to the value. The manner in which a stone is cut may add to its value,
either because of the light effects or because of the interesting
shape. One might expect perfection, freedom from flaw, to be important,
and indeed in the diamond this is so. The most common flaw in the
diamond, by the way, is not a crack but a speck or tiny specks of carbon
remaining between the crystals, the diamond being a crystallized form
of carbon. In other stones—as we have observed of the star ruby, the
star sapphire and the cat’s eye—a physical flaw may result in a greater
aesthetic desirability. Other special features may enhance the value of a
particular stone; a recently discovered ruby is the only known example of
a double star, with not six but twelve rays. The history and associations
of a gem or jewel, dramatic or sentimental, storied or personal, may be
what makes its possession desirable.
_Measurement_
One seldom speaks of the size of a precious stone; other things being
equal, its value is estimated by its weight. The unit of weight, in
measuring precious stones, is the carat. As the word carat comes from
the Arabic, meaning the nut or bean of the carob tree, it was evidently
in olden times a rather imprecise measure. It has now been made definite
as two-tenths of a gram (1c. = 0.2 gr.). It takes 141¾ carats to make an
ounce, and therefore 2,268 carats to make a pound. Smaller diamonds are
measured by points; one hundred points equal one carat.
The pearl is usually measured by the grain; a grain equals ¼ of a
carat, or one twentieth of a gram (0.05 gr.). (This grain is not to
be confused with the grain that is the smallest unit in the English
system of weight.) Any pearl which is less than one quarter of a grain
is called a seed pearl; an ounce of these may contain as many as 7,000
to 9,000 pearls. They are used in embroidery, in weaving cloth, and for
many-stranded chains.
_The Precious Metals_
The purity of gold is also measured in carats; in the United States,
to distinguish the two systems, the gold weight is spelled with a _k_:
karat. Pure gold is spoken of, arbitrarily, as being 24 karat gold. Pure
gold, however, is too soft for most uses, especially in jewelry; it is
therefore mixed with a harder metal; the mixture, and the less valuable
metal used in the mixture, are both called the alloy. The number of
karats of gold indicated is the proportion of pure gold in the alloy.
Thus, 18 karat gold means 18 parts of pure gold mixed with 6 parts of
alloy.
_Alloys_
The alloy is usually formed by fusing metals together; when molten
they dissolve in each other and form an intimate union, often (as in
industrial uses) producing a new metal with qualities quite different
from those of the separate elements of the mixture. The admixture of
nickel or zinc with gold produces what is called white gold; an alloy of
copper or brass is red gold, ranging in color from pink to deep rose; an
alloy of silver is green gold. In addition to gold—mainly 20, 18, and 14
karat gold—pure (sterling) silver, platinum and, more recently, palladium
are also effectively employed for jewels, alone or as background in
which to set precious stones. Other precious metals occasionally used in
the making of jewels are iridium, rhodium and ruthenium. The favorites,
however, continue to be platinum and gold.