Natural Marconi Obsidian Smooth Hand Polished Pear Drop Briolette Beads Strand Rondelles Sold per 4 inches strand & Sizes from 7mm approx. Labradorite a feldspar mineral, is an intermediate to calcic member of the plagioclase series. Labradorescence is a side-effect of the molecular change which occurs in large crystal masses of anorthosite, producing an iridescent play of colors.