Garnet

Kathy Feick

Garnet is the name given to a group of chemically and physically similar silica minerals. The history of garnet dates back to the Bronze Age, more than 5000 years ago. Its name, which has been used since these ancient times, was derived from the Latin word granatium, which means pomegranate (because the small, red crystals were thought to resemble pomegranate seeds). The original name was actually granat, but in time the name morphed into the name garnet.

orange garnet crystals in asbestos matrix

Garnet on Asbestos. Unknown Locality. University of Waterloo Earth Sciences Museum Collection.

Garnet is relatively common in highly metamorphosed rocks and in some igneous rocks. Because this group of minerals forms under high temperatures and pressures, the surrounding rock is left with an imprint of this great stress. Because of this, garnet may be used by geologists to gauge how much temperature and pressure the surrounding rocks endured.

Garnets are isostructural, meaning they all share the same crystal structure. This leads to similar crystal shapes and properties. Garnets generally produce symmetrical, cube-based crystals. The most common crystal shape among this group is the rhombic dodecahedron, a twelve-sided crystal with diamond shaped (rhombic) faces. The basic shape is a trademark for garnets, for no other crystal shape is so closely associated with a single mineral group.

dark red-green coloured garnet crystal exhibiting rhombic shape

Garnet Crystal, Unknown Locality. University of Waterloo Earth Sciences Museum Collection.

Different varieties:

Different varieties of garnet contain different metal ions, such as iron, aluminum, magnesium, chromium, or calcium in their structure.

Almandine: This is the most common garnet found. It is red, reddish-brown or black in colour. It is also the hardest form of garnet with a value of 7.5-8 on Moh’s hardness scale.

red garnet crystals in white rock

Almandine Garnet, Northern Pakistan. University of Waterloo Earth Sciences Museum Collection.

Pyrope: Pyrope is the most well-known gemstone form of garnet and has a red colour. It is softer than Almandine with a hardness of 7-7.5 on Moh’s hardness scale.
Rhodolite: This is a red-rose form of garnet. It has a lighter tone than Almandine and Pyrope and is more purple in colour. It is actually an intermediate variety garnet with a composition somewhere between pyrope and almandine. It usually contains more magnesium than iron in its chemical structure.
Spessartite: Spessartite is commonly orange to orange-red in colour, though it may also be red, yellow, brown or pink. The colours are generally caused by iron impurities. Spessartite has an exceptionally high refractive index, giving it a special brilliance.
Grossular: Pure grossular is colourless. Due to a number of impurities however, it has the widest range of colours than the other garnets. It can be white, colourless, red, green, yellow, orange, orange, brown, pink, purple or gray.

orange garnet

Grossularite variety: Hessonite. Asbestos, Quebec. University of Waterloo Earth Sciences Museum Collection.

Tsavorite: Tsavorite is the trade name for the emerald-green variety of garnet. It resembles emerald though it is much rarer and is often flawless and free of inclusions. The green colour may be caused by chromium or vanadium impurities in its structure.

small green crystals in white matrix

Grossularite variety: Tsavorite. North Creek, New York, USA. University of Waterloo Earth Sciences Museum Collection.

Andradite: Andraite is the most lustrous of the garnets. It can be red, green, yellow, orange, brown, pink, gray, black or multicoloured.
Urarovite: Urarovite is the rarest of the familiar garnets. It only occurs in very small crystals and thus, is seldom used as a gem. It is a brilliant deep chrome-green colour.

Uses:

Gemstones (thought a very small number are pure and flawless enough to be cut into gemstones)
Sanding belts, paper, discs and strips
Abrasive blasting mineral
Water filtration
Abrasive powder