Dogtooth spar

Dogtooth spar is a speleothem found in limestone caves that consists of very large calcite crystals resembling dogs' teeth (hence the name). They form through mineral precipitation of water-borne calcite. Dogtooth spar crystals are not limited to caves, but can grow in any open space including veins, fractures, and geodes.
These sharp tooth-shaped crystals are generally of the magnitude of centimeters long, but anomalous samples decimeters long exist, notably in Sitting Bull Crystal Caverns. A layer of crystalline calcite can be found underneath the surface of crystal points.
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 [211].
Spar is a general term for transparent to translucent, generally light-colored and vitreous crystalline minerals.