feldspar - any of a group of hard crystalline minerals that consist of aluminum silicates of potassium or sodium or calcium or bariumatomic number 14, silicon, Si - a tetravalent nonmetallic element; next to oxygen it is the most abundant element in the earth's crust; occurs in clay and feldspar and granite and quartz and sand; used as a semiconductor in transistors orthoclase - a white or colored monoclinic feldspar moonstone - a transparent or translucent gemstone with a pearly luster; some specimens are orthoclase feldspar and others are plagioclase feldspar spar - any of various nonmetallic minerals (calcite or feldspar) that are light in color and transparent or translucent and cleavable |