November 29, 2010


: David K. Lynch
Summary Author: David K. Lynch

Garnets are a family of neosilicate minerals with a distinctive, though somewhat variable, composition and similar crystalline properties. All garnets form as hard (6 – 8 on the Mohs scale of hardness) isometric crystals, typically rhombic dodecahedrons. Most garnets are reddish-brown, though they occur in many colors. Garnets are primarily metamorphic in origin and vary from microscopic to a few centimeters in diameter. The largest single garnet crystal ever found is an astounding 7.5 ft (2.3 m) in diameter and is on display in the Natural History Museum, Bergen, Norway. When formed in schist garnets nucleate and grow suspended in the hot viscous rock, so every facet is well defined. These make attractive specimens because of the absence of any attachment damage such as with crystals from a vug or geode. Massive deposits of garnets are ground up and used as abrasives – hence the brown color of sandpaper. Exceptionally clear or colored garnets are used as gemstones.

The garnets pictured here came from a deposit of garnet schist (upper part of the picture) in Baja, Mexico. They are almandine garnets and are near perfect rhombic dodecahedrons (lower right). Most of these were covered with a rusty deposit that was easily scraped off. This is one of the richest deposits of garnets I've ever seen; in some places, garnets comprised more than half of the volume of the schist. So densely packed were the garnets that some formed in clusters as seen at the lower left. A US penny is included for size comparison.

Photo Details: Camera: Pentax K200D; Focal Length: 130.0mm (35mm equivalent: 195mm); Aperture: f/19.0; Exposure Time: 0.0080 s (1/125); ISO equiv: 800.