in natural history, is the name of a genus of crystals, derived from the Greek πολύς, many, ἀπό, from, the primitive particle ἀ-, not, and στύλος, a column; and means a crystal with many planes, and without a column.

The bodies of this genus are crystals of two octagonal pyramids, with the bases joined, the whole body consisting of 16 planes. Of this genus there are only two species known: 1. A brown kind with short pyramids, found in great plenty in Virginia on the sides of hills; and, 2. A colourless one, with longer pyramids. This has yet been found only in one place, which is the great mine at Gofflaer, in Saxony, where it usually lies at great depths.