belonite

from The Century Dictionary.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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