Word Wednesday: Conchoidal

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

Conchoidal (kon-KOY-dul) describes a curved, shell-like fracture surface. The name comes from a word for shell.

Fracture versus cleavage

Minerals with cleavage tend to break repeatedly along flat structural planes. Conchoidal fracture curves instead, often producing ripples and very sharp edges. Quartz commonly fractures this way because it lacks cleavage. Obsidian and manufactured glass also show strong conchoidal fracture.

Why it matters

Fracture is one clue in identification. A glassy luster plus conchoidal fracture might suggest quartz, glass, or obsidian, but hardness, inclusions, geological context, and other tests are still needed.

Safety first

Fresh conchoidal edges can be sharper than they look. Do not deliberately break an unknown or valuable specimen for a test. Handle broken glassy material with gloves, protect your eyes, and package sharp edges securely.

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.