Clear quartz is a purification stone for all the black gemstones. Black tourmaline gemstones are also available in various traditional and fancy shapes and a range of cutting styles. Onyx is a translucent to semitransparent variety of agate which in turn is a variety of chalcedony or cryptocrystalline quartz that is porous and consists of straight or parallel banded agate of alternating shades of milky white and black gray and black black and red white and red white and brown.