San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide
[Grossular example of garnet. Collection of the San Diego Natural History Museum.]

Garnet

Probably from the Latin, granatum malum, meaning "pomegranate."

Description and Occurrence

Most garnet crystals form as small, chunky balls, with many symmetrical sides. They come in all colors except blue. Gemstone-quality garnets -- almandine, pyrope -- can be faceted or shaped into cabochons. They are highly prized for their brilliance, hue, and transparency. However, many forms of garnet are not gem-quality crystals; these are generally used as abrasives for grinding and polishing.

Garnet usually occurs in metamorphic rock and is common all over the world. It can be found in Australia, Brazil, Sri Lanka, and the Czech Republic. In the United States, gem-quality stones have been located in Arizona, Utah, Alaska, Idaho, and North Carolina. Pegmatites in the Ramona area of San Diego County have yielded crystals as large as 2.5 cm.

Key members of the garnet group are almandine (red to violet-red), spessartine (yellow, rose, or orange to reddish-brown), pyrope (deep red), grossular (white, yellow, yellowish-green, brownish-red, orange, or black), and andradite (colorless, yellow-green, or brown to black). Andradite includes the prized garnet, an emerald green variety called demantoid. The Museum has an example of demantoid in its Hall of Mineralogy.

Field Notes: Best field marks for garnet is the crystal shape (sometimes described as little soccer balls) and hardness.


Physical Properties

Color Streak Transparency Luster Hardness Cleavage Fracture Specific gravity Crystal form
colorless, all colors except blue colorless transparent to opaque vitreous 6.5 to 7.5 none conchoidal, brittle 3.4 to 4.6 isometric

Photo: Garnet, grossular form. Collection of the San Diego Natural History Museum
Photo credit: Linda West

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