belonite

Century Dictionary and Cyclopedia

1. n. A kind of minute imperfect crystals, usually acicular in form, sometimes dendritic, observed in glassy volcanic rocks. The term is now limited to such as exert no action on polarized light.

Wiktionary

1. n. mineralogy One of the minute acicular or dendritic crystalline forms sometimes observed in glassy volcanic rocks.

GNU Webster's 1913

1. n. (Min.) Minute acicular or dendritic crystalline forms sometimes observed in glassy volcanic rocks.