A whitish mineral consisting of sodium aluminum silicate or potassium aluminum silicate in crystalline form; used in the manufacture of ceramics and enamels WordNet 3.0 & Open English WordNet

Nepheline, also called nephelite, is a rock-forming mineral in the feldspathoid group – a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites. Wikipedia

A feldspathoid mineral of silica-poor igneous, plutonic and volcanic rocks. Chemically, nepheline is a plagioclase feldspar with insufficient silica to satisfy the chemical bonds. Because of the unfilled bonds, nepheline weathers rapidly and can only be seen as inclusions in freshly broken rock. Wiktionary

Feldspathoid mineral. Wiktionary (translation)