Dogtooth spar

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Dogtooth spar deposited on the walls of a part of the Caverns of Sonora near Sonora, Texas. The former water level can be seen in the upper left hand corner.

Dogtooth spar is a speleothem found in limestone caves that consists of very large calcite crystals resembling dogs' teeth (hence the name). They are usually found near standing water, where crystals can grow for a very long time. Most caves with dogtooth spar have seasonal flooding.

Dogtooth spar crystals are not limited to caves, but can grow in any open space including veins, fractures, and geodes. The sharply tooth-shaped crystals typically consist of acute scalenohedrons, twelve triangular crystal faces that ideally form scalene triangles. However, modification of these faces is common, and individual crystal faces may have many more than three edges. Calcite crystallizes in the rhombohedral system, and the most common scalenohedron form has the Miller index $[21\bar{3}1]$ .