A double terminated crystal is a crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 with two naturally facet

Facet

Facets are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure...

ed ends. It is a rarer form of crystal as it forms free-floating in pockets of clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

, rather than on one side of a stone. They tend to be highly sought-after, as those who believe in the power of crystals claim that energy can flow in both directions through a double terminated crystal, and that this makes them ideal for ritual "healing" ceremonies and astral projection

Astral projection

Astral projection is an interpretation of out-of-body experience that assumes the existence of an "astral body" separate from the physical body and capable of traveling outside it...

, among other applications. They are often made into pendula

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

or pendants.