Garnet is the name which can be applied to six similar mineral species,
namely almandine, pyrope, spessartine, grossular, andradite and uvarovite.
To further complicate matters, many garnets are actually a combination of
these minerals. Rhodolite garnet for instance, is a combination of almandine
and pyrope, and is sometimes referred to as pyrope-almandine garnet. There
are also many trade names and other commonly used names which only adds to
the confusion, such as Rhodolite, Tsavorite, Hessonite, Malaya, Mozambique,
Mandarin, Ant-hill, Leuco, Hydrogrossular, Demantoid, Melanite, Topazolite,
Thai. Other names such as "cape ruby" are simply misleading and deceptive.
Some garnets also exhibit color change and stars.
Garnet displays the greatest variety of color of any mineral, occurring in
every color except blue. For example, grossularite can be colorless, white,
gray, yellow, yellowish green, various shades of green, brown, pink,
reddish, or black. Andradite garnet can be yellow-green, green, greenish
brown, orangy yellow, brown, grayish black or black. Pyrope is commonly
purplish red, purplish red, orangy red, crimson, or dark red; and almandite
is deep red, brownish red, brownish black or violet-red. Spessartite garnet
can be red, reddish orange, orange, yellow-brown, reddish brown, or blackish
brown. A few garnets exhibit a color-change phenomenon. They are one color
when viewed in natural light and another color when viewed in incandescent
light.
Varieties:
Rhodolite- violet to purplish-red;
Almandite - red, brownish-red, violetish-red or Purple;
Pyrope - red;
Grossularite - green, yellow, brown, white, colourless, light violet, red,
orangey-red;
Varieties: hessonite (orange to brown), transparent, green, grossularite
(tsavorite); Some show a colour change from a mauve-brown to orange-red.
Andradite - green, yellow, black. Green called demantoid (high lustre and
dispersion); Spessartite - yellow to yellow-brown, dark orangey-brown,
reddish-orange, orange; Uvarovite - emerald green, found only in tiny sizes,
usually opaque.
Sources:
Rhodolite - Sri Lanka, North Carolina, Tanzania, Kenya, South Africa,
Brazil.
Almandite - Sri Lanka, India, Brazil, star from Idaho - USA.
Pyrope - Czechoslovakia, South Africa, Zimbabwe - Rhodesia, Brazil, Arizona.
Grossularite - Sri Lanka, Brazil, Tanzania, Kenya, South Africa, Canada.
Andradite - demantoid: Russia, Italy; translucent yellowish or
greenish-brown, Arizona.
Spessartite - Sri Lanka, Burma, Brazil, Madagascar, Tanzania, Kenya.
Uvarovite - Russia, Finland (hardly mined at all).
History - Since earliest times garnets have been carried as amulets against
accidents in travel. Asiatic peoples and even our Southwest Indians used
them as bullets, believing that their rich, glowing colour might cause more
deadly wounds. The Persians have given the garnet a favoured place as a
royal stone, allowing it to bear their sovereign's image. Red garnet was
once used to relieve fever, yellow garnet to cure jaundice. If the powder
failed, the apothecary was accused of using a substitute.
The use of garnets as a gem or gemstone can be traced to prehistoric times.
However, the first industrial use of garnet appears to have been as coated
sandpaper manufactured in the United States by Henry Hudson Barton (founder
of Barton Mines Corp.) in 1878. Its use has grown from that single sample of
garnet coated sandpaper, to world industrial uses of more than 110,000 tons
per year. In 1994, United States production of industrial garnet was valued
at about $14 million, while gem garnet production was valued at only about
$233,000.
Garnets are isostructural, meaning that they share the same crystal
structure. This leads to similar crystal shapes and properties. Garnets
belong to the isometric crystal class, which produces very symmetrical,
cube-based crystals. The most common crystal shape for garnets however is
the rhombic dodecahedron, a twelve sided crystal with diamond-shaped
(rhombic) faces. This basic shape is the trademark of garnets, for no other
crystal shape is so closely associated with a single mineral group like the
rhombic dodecahedron is with garnets. Most garnets are red in color, leading
to the erroneous belief that all garnets are red. In fact a few varieties,
such as grossular, can have a wide range of colors, and uvarovite is always
a bright green. As a mineral specimen, garnets usually have well shaped and
complex crystals and their color and luster can make for a very beautiful
addition to a collection. At times, garnets are accessory minerals to other
valuable and pretty gem minerals such as topaz, beryl, tourmaline,
vesuvianite and diopside making these specimens extra special.
There is a misconception that garnets are only a red gem but in fact they
come in a variety of colors including purple, red, orange, yellow, green,
brown, black, or colorless. The lack of a blue garnet was remedied in 1990's
following the discovery of color-change blue to red/pink material in Bekily,
Madagascar but these stones are very rare. Color-change garnets are by far
the rarest garnets except uvarovite, which does not come in cuttable sizes.
In daylight, their color can be shades of green, beige, brown, gray and
rarely blue, to a reddish or purplish/pink color in incandescent light. By
composition, these garnets are a mix of spessartine and pyrope, as are
Malaya garnets. The color change of these new garnets is often more intense
and more dramatic than the color change of top quality Alexandrite which is
frequently disappointing, but still sells for many thousands of dollars (US)
per carat. It is expected that blue color-change garnets will match
Alexandrite prices or even exceed them as the color change is often better
and these garnets are much rarer.
Six common varieties of garnet are recognized based on their chemical
composition. They are pyrope, almandine or carbuncle, spessartite,
grossularite (varieties of which are hessonite or cinnamon-stone and
tsavorite), uvarovite and andradite. The garnets make up two solid solution
series; 1. pyrope-almandine-spessarite and 2.
uvarovite-grossularite-andradite.
Garnet is the birthstone for January, and has been used since the Bronze
Age.
Pure crystals of garnet are used as gemstones. Garnet sand is a good
abrasive, and a common replacement for silica sand in sand blasting. Pyrope
varieties are used as kimberlite indicator minerals in diamond prospecting.
Garnets are very abundant in the lower crust and mantle and thus play an
important role in geochemical understanding of the Earth.
