These are euhedral crystals of one of the rarest members of the plagioclase feldspar family, typoical but rare for the locality on the market these days - I have only seen a few in overa decade since seeing the first one. These crystals are opaque white and reach 1 cm across. From mindat: This locality is a small island with active volcano. Augite-olivine basalt. Famous as the classic locality for anorthite crystals, thousands of which were thrown out loose by the 1870 eruption and distributed in collections around the world; usually coated with a film of reddish or blackish glassy lava.