Spar is a general term used to refer to crystals where the crystal faces are readily discernible. In caves, spar is a depositional deposit, usually made of calcite or gypsum, but sometimes of less common (to caves) minerals such as barite, fluorite, halite, or quartz. Most spar forms underwater, either in the phreatic zone (below the water table, where most caves are formed), or in shallow standing pools, as pool spar. Spar may also grow in the air from solutions seeping out of the cave walls or through porous sediments.

The largest spar, like that seen in photos 1 and 2 below, is of the phreatic variety, since it usually has more time to grow. It grows best where the water is just barely saturated. Some caves are like giant geodes, the walls and ceilings completely lined with spar, such as Jewel Cave and others in the Black Hills of South Dakota. The second photo is nailhead spar from Jewel Cave. The third photo is so-called dogtooth spar from a room in an undeveloped portion of Carlsbad Cavern.
Spar formed in the air is often made of gypsum or selenite as it is known in its crystalline form. It may form as thin needles found in sediments, or on the growing tips of gypsum chandeliers, as shown in the third photo.

AUTHOR: Dave Bunnell