Adirondack Garnet

Adirondack Garnet is almandine garnet from the Adirondack Mountains of New York, USA, most famously from Gore Mountain near North Creek. It is the official New York State gem and the foundation of the historic Barton Mines, a leading producer of industrial garnet abrasive.

The garnets here are notable for their enormous size — some crystals reach a foot or more across — set in a dark host rock. The garnet is typically deep red to reddish-brown, hard, and tough, with the characteristic fracturing that makes it valuable as an abrasive.

While mostly industrial, well-formed and clearer pieces are collected as specimens and occasionally cut.

Gore Mountain garnet formed during high-grade regional metamorphism (granulite facies) of the ancient Adirondack rocks over a billion years ago. The garnets grew as large crystals (porphyroblasts) in a metagabbro/amphibolite host, often surrounded by rims of black hornblende.

The exceptionally large crystal size reflects slow growth under sustained high temperature and pressure, with fluids aiding crystallization. The combination of size, hardness, and the way the garnet fractures into sharp, angular pieces makes Gore Mountain almandine ideal for abrasives.

Adirondack Garnet appears as large, deep red to reddish-brown almandine crystals, commonly rimmed with black hornblende in the host rock.

• Hardness: 7-7.5.
• Streak: white.
• Optic character: singly refractive.
• Fracture: conchoidal to splintery, no cleavage.
• Density: high.

The huge crystal size, dark red color, and black amphibole rims in the Gore Mountain rock are highly diagnostic. Single refraction and high density separate garnet from other red minerals.

Adirondack Garnet is overwhelmingly an industrial mineral, crushed into abrasive for sandpaper, waterjet cutting, sandblasting media, and grinding. Its hardness and sharp-fracturing nature make it among the best natural abrasives.

Larger, cleaner pieces are sold as mineral specimens and the famous Gore Mountain garnet-in-matrix is a collector favorite. Faceting-grade material is limited because the garnet is often heavily included.