Two of the more famous stones in the British crown jewels are the Black
Prince’s Ruby and the Timur Ruby, but neither of these stones is really
ruby. Like the great red gem in the crown that belonged to the Russian
Empress Catherine II, these two British stones are spinel. Although
spinel occurs in many colors, such as yellow, green, violet, brown, and
black, it is the red spinel that usually is seen in the gem trade. There
are several varieties of red spinel, such as _ruby spinel_, _balas
ruby_, _rubicelle_, and _almandine spinel_—all of which refer to the
color resemblance to ruby.
[Illustration: The hues and tints of spinel show subtle variations
that are matched only by those of tourmaline. Unlike tourmaline,
however, spinel may be bright ruby red. The cut stones curving
around two pieces of rough from Burma weigh (left to right) 30
carats (Ceylon), 34 carats (Burma), 36 carats (Burma), 30 carats
(Ceylon), and 22 carats (Ceylon). (Three-fourths actual size.)]
Spinel is an oxide of magnesium and aluminum, and it is not related to
ruby. However, because its hardness (8) is only slightly less than that
of ruby and its brilliance is about equal to that of ruby, spinel makes
an excellent substitute for that gem. Also, because it is more
plentiful, spinel costs much less. It is interesting that red spinel,
like ruby, gets its color from the presence of traces of chromium.
Synthetic blue spinel is widely used as a substitute for aquamarine, and
synthetic spinels of other colors are used as substitutes for many gems.
However, the synthetic stones are not ordinarily made in the subtle
shades so characteristic of natural spinel. Completely colorless spinel
apparently exists only as a synthetic material. Actually, because of its
hardness, durability, and many attractive colors, spinel makes a fine
gemstone in its own right.
Like ruby and several other gemstones, spinel is found chiefly in the
gem gravels of Ceylon, Burma, and Thailand. Appreciable amounts of
spinel occur in the Ceylon gem gravels as worn, rounded pebbles of many
colors. In the Burmese gravel deposits the spinel is often found as
well-formed octahedral crystals. Near Moguk, in Burma, spinel has been
found in its original position in the limestone rocks as well as in the
eroded stream deposits.
VARIETIES
Almandine spinel: Purplish red
Rubicelle: Orange-red
Balas ruby: Rose red
Ruby spinel: Deep red
Chlorospinel: Translucent grass green
Ceylonite or pleonaste: Opaque dark green, brown, or black
Picotite or chrome spinel: Translucent dark yellow-brown or
green-brown
QUARTZ
(INCLUDES ROCK CRYSTAL, AMETHYST, AND CITRINE)
Few gemstones can compete with quartz for variety of color. Having a
hardness of 7 and occurring in many beautiful varieties, only the
relative abundance of quartz prevents the species from attaining top
rank among gemstones.
The two kinds of quartz, crystalline and cryptocrystalline
(fine-grained) quartz, occur in all kinds of mineral deposits throughout
the world. Much of this material is suitable for cutting gems.
Colorless crystalline quartz, or _rock crystal_, makes attractive
faceted gems, and it is used as a suitable substitute for diamond and
zircon even though it lacks the fire and brilliance of those gemstones.
Some very large, flawless crystals of colorless crystalline quartz have
been found. The great Warner Crystal Ball, with a diameter of 12⅞ inches
and weighing 106¾ pounds, was cut from such a crystal. In addition to
the name rock crystal, colorless crystalline quartz appears in the
jewelry trade under such names as rhinestone (not to be confused with
the glass substitute), Herkimer diamond (from Herkimer County, N. Y.),
and Cape May diamond (from Cape May, N. J.).
The most popular variety of quartz is _amethyst_, a transparent form
whose color ranges from pale violet to deep purple. In many cut stones
of amethyst the color intensity changes sharply from section to section.
This is due to irregular color zoning common to amethyst crystals. The
actual cause of the purple color in amethyst is not very well
understood. There are fewer cut stones of amethyst in very large sizes
because of the rarity of large, flawless, well-colored crystals.
[Illustration: This 4500-carat pale smoky quartz egg from California
rests on a gold stand set with Montana sapphires. The unique gem was
cut and its stand was designed and made by Capt. John Sinkankas as a
difficult exercise in the lapidary art. The quartz egg is 4 inches
long and almost 3 inches in diameter.]
The name _citrine_ (from the French word for lemon) attempts to describe
the yellow color of another variety of quartz. Actually, the normal
coloring of citrine varies from yellow to red-orange and red-brown, but
the yellow sometimes rivals the yellow of topaz. In addition to the
normal color range, the colors of citrine may grade through a grayish
yellow variety known as _cairngorm_ and a grayish variety called _smoky
quartz_ to a black variety called _morion_. Other varieties that add
color dimensions to the group of quartz gemstones are _rose quartz_ and
_milky quartz_. Like amethyst, the reason for the color in rose quartz
has not been definitely established. Milky quartz owes its color to
myriads of tiny cavities containing water or liquid carbon dioxide.
[Illustration: A 783-carat step-cut citrine of deep, rich color
dwarfs a 278-carat brilliant-cut citrine (at left), a 90-carat smoky
quartz, and a 91-carat briolette of citrine. The smoky quartz, from
Switzerland, is so dark that it appears to be opaque. The other
three stones came from Brazil. The briolette and brilliant-cut
citrines were cut and donated to the Smithsonian Institution by
Albert R. Cutter. (Slightly less than half actual size.)]
The range of color in quartz is somewhat surprising, considering that
the mineral is a simple silicon dioxide. Some of the colors, as with
corundum and some other gemstones, are due to traces of impurities. In
quartz, these consist mainly of oxides of iron, manganese, and titanium.
However, all the reasons for quartz coloration in its many varieties are
not known.
[Illustration: Pastel rose quartz has a delicate beauty in any cut.
The 375-carat step cut (top), the 84-carat step cut, and the
46-carat marquise came from Brazil. (Two-thirds actual size.)]
[Illustration: Amethyst, a purplish quartz, is the birthstone for
February. Here it is represented by a 1362-carat stone from Brazil
(top), a 54-carat stone from Pennsylvania (left), and a 21-carat
stone from North Carolina. (Almost actual size.)]
In addition to possessing wide variation of color, quartz, like sapphire
and certain other gemstones, can exhibit asterism or chatoyancy. The
well-known _tiger’s-eye_ from West Griqualand, South Africa, owes its
eye effect to the fact that its material is a replacement of fibrous
asbestos by cryptocrystalline quartz. The color of tiger’s-eye arises
from the partial alteration of the asbestos to yellow-brown iron oxides
before it is replaced by quartz. Inclusions of rutile, tourmaline, or
actinolite needles may produce attractive patterns in quartz, but they
do not always cause chatoyancy. The material containing such inclusions
is called sagenitic quartz, or it may be descriptively named, such as
rutilated quartz, tourmalinated quartz, and so forth. Sagenitic quartz
is usually cut as cabochons rather than as faceted stones since the
inclusions are of greater interest than the quartz itself.
If the foreign inclusions consist of tiny flakes of hematite or mica,
the quartz assumes a spangled appearance and is called _aventurine_.
Crystals of quartz varieties that are opaque or that contain visible
inclusions normally are cut as cabochons to take advantage of the body
color or to make the inclusions more visible. Crystals of the
transparent varieties are fashioned in any of several cutting styles,
depending on whether it is desired to take maximum advantage of color or
of brilliance. Because of its availability in fairly large, flawless
pieces in various colors, quartz has been used extensively in carving.
The Chinese have excelled in carving large, ornate objects of rock
crystal.
Although quartz occurs in many varieties and its crystals are cut in
many styles, it is easily identified by its refractive index of 1.55,
specific gravity of 2.65, and hardness of 7.
CRYSTALLINE VARIETIES
Amethyst: Purple to violet
Cairngorm: Smoky yellow
Citrine: Yellow to red-orange and red-brown
Milky quartz: White
Morion: Black
Rock crystal: Colorless
Rose quartz: Rose to pink
Smoky quartz: Gray to black
CRYPTOCRYSTALLINE VARIETIES (CHALCEDONY)
Agate: Pronounced color banding
Aventurine: Inclusions of sparkling flakes
Bloodstone: Dark green dotted with red
Carnelian: Red to yellow-red
Cat’s-eye: Chatoyant
Chrysoprase: Green
Jasper: Opaque brown to red-brown, green, yellow, etc.
Onyx: Color banding in straight layers of contrasting color
Sard: Light to dark brown
Sardonyx: Sard or carnelian bands alternating with white bands
Tiger’s-eye: Bright brownish yellow, sometimes blue: chatoyant
CHRYSOBERYL
(INCLUDES ALEXANDRITE AND CAT’S-EYE)
With color ranging from shades of yellow and brown through blue-green to
olive, and with a hardness of 8½, chrysoberyl has most of the
characteristics necessary for a fine gem. Rare stones of high-quality
chrysoberyl demand fairly high prices, and they are sought eagerly by
the connoisseur of gemstones.
Chrysoberyl is beryllium aluminate, and thus is closely related to the
gemstone spinel, which is magnesium aluminate. When pure, chrysoberyl is
colorless and relatively uninteresting as a gemstone because of its lack
of color dispersion and its moderate refractive index of 1.75. However,
few pure samples are known, as chrysoberyl normally contains some iron
or chromium in place of aluminum and some iron in place of beryllium. As
a result of such impurities, the color of chrysoberyl my be yellowish,
greenish, or brownish.
Chrysoberyl and beryl are the only important gemstones containing the
element beryllium. The minerals beryllonite, euclase, hambergite, and
phenakite also contain this element, but they are rare and seldom are
seen as cut gems.
[Illustration: One of the finest chrysoberyl cat’s-eyes in existence
is the 58-carat Maharani from Ceylon. (Actual size.)]
The _alexandrite_ variety of chrysoberyl has two colors in delicate
balance, and it changes from a columbine red to an emerald green when
viewed under different light. When viewed in daylight, which is richer
in green, the color balance shifts toward green, and that hue is seen by
the observer. Under artificial light, normally richer in red, the color
balance shifts toward red, and the stone seems to have changed to that
color. This extremely rare stone, named after Czar Alexander II of
Russia, is found only occasionally, in Russia and Ceylon. The Russian
stones, found with emerald in mica schist, tend to be smaller than the
Ceylon stones and have a color change going from emerald green to
violet-red. The Ceylon stones, found as pebbles in gem gravels, have a
color change going from a less-emerald green to a browner red. The
66-carat, record-size alexandrite in the National Collection shows the
color change typical of Ceylon stones. A synthetic stone is commonly
marketed as synthetic alexandrite, but this substitute not only is
man-made but is actually synthetic corundum instead of synthetic
chrysoberyl.
[Illustration: In addition to its fine cat’s-eyes and its
color-changing alexandrite varieties, chrysoberyl occurs in handsome
stones that vary in depth of color. Shown here with an uncut twinned
crystal of gem quality from Brazil are a 46-carat stone from Brazil
(left) and a 121-carat stone from Ceylon. The uncut crystal is a
gift of Bernard T. Rocca, Sr. (Two-thirds actual size.)]
_Cat’s-eye_ chrysoberyl contains myriads of tiny fiberlike channels
arranged in parallel position. When the stone is cut as a cabochon, a
band of light is reflected from the curved top of the stone, producing
an effect that resembles the slit pupil of a cat’s eye.
VARIETIES
Alexandrite: Green in daylight, changing to red in artificial light
Cat’s-eye: Chatoyant