Garnet

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The most common color is red, but garnets can be found in almost every color, including orange, yellow, green, purple, brown, blue, black, pink, and colorless. The rarest of these is the blue garnet.

Formation & Geology

Garnets are typically found in metamorphic rocks (schist, gneiss) and some igneous rocks (granite, kimberlite). They form under high temperatures and pressures. Because garnet is so resistant to weathering, it can also be found in sedimentary rocks like sandstone and conglomerate. Almandine is the most common garnet, often found in mica schists.

Physical Characteristics

Garnet species share the same crystal structure (cubic) but vary in chemical composition. They generally have a hardness ranging from 6.5 to 7.5. They have no cleavage, meaning they break with irregular or conchoidal fractures. This property, combined with their hardness, makes them excellent abrasives (e.g., garnet sandpaper).

Historical Significance

The word garnet comes from the 14th‐century Middle English word gernet, meaning 'dark red'. It is derived from the Latin granatus, from granum ('grain, seed'). This is possibly a reference to pomum granatum ('pomegranate'), a plant whose fruits contain abundant and vivid red seed covers (arils), which are similar in shape, size, and color to some garnet crystals. It is the birthstone for January.

The Garnet Group Family

Garnet is not a single mineral but a group of several closely related minerals that share the same crystal structure but have different chemical formulas. The main members are Pyrope (red), Almandine (red/purple), Spessartine (orange), Grossular (green/yellow), and Andradite (green/black). The rarest and most valuable is the brilliant green Demantoid garnet.