Word Wednesday: Asterism

Welcome to Word Wednesday, Wolfstone Rock Shop’s weekly look at the language of rocks, minerals, and gems.

Asterism (ASS-ter-iz-um) is a star-shaped optical effect that appears on the surface of certain gemstones under a concentrated light.

How a star forms

Sets of aligned needle-like inclusions or microscopic structures reflect light in intersecting bands. A cabochon cut and careful orientation bring those bands together. Two sets may make a four-rayed star, three sets a six-rayed star, and other arrangements can create more rays.

Where it appears

Star sapphire and star ruby are famous examples, but asterism can also occur in some garnet, quartz, diopside, and other materials. The sharpness, centering, movement, body color, and overall condition all affect appeal.

Collector tip

Inspect a star stone under a single-point light and move the light or stone. The star should move across the curved surface in a way consistent with its internal structure. Treatments and synthetic star stones exist, so an optical effect alone does not establish origin.

See this texture and other mineral features in person at Wolfstone Rock Shop, 1857 N Casaloma Dr, Appleton, WI 54913. Inventory changes often, so call (920) 809-8577 if you are looking for a particular example.