oligoclase — variety of albite; data; see oligoclase in ‘Classification’ …
olivine — a series between forsterite and fayalite …
oolite — (egg stone); a sedimentary rock formed from spherical grains composed of concentric layers …
opal — species of mineral; see Opal in ‘Classification’ …
opalescence — a type of dichroism seen in opals; gems appear yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light …
opaque — clarity grade in gemstones; does not allow light to pass through, highly included; some gems are naturally opaque …
Oregon sunstone — variety from anorthite-albite series; see Oregon sunstone in ‘Classification’ …
orlets — variety of rhodonite; see orlets in ‘Classification’ …
orthoclase — species of mineral; see orthoclase in ‘Classification’ …
oval — a commonly used shape in faceting a gemstone …
painite — species of mineral; data; faceted specimens exist …
pargasite — species of mineral; see pargasite in ‘Classification’ …
pavilion — the bottom part of faceted stone …
pear — a commonly used shape in faceting a gemstone …
pectolite — species of mineral; see pectolite in ‘Classification’ …
pegmatite — crystalline intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm …
pentagon — rare shape in faceting a gemstone …
peridot — variety of forsterite; see peridot in ‘Classification’ …
peridotite — a dense, coarse-grained igneous rock, consisting mostly of olivine and pyroxene …
petalite — species of mineral; see petalite in ‘Classification’ …
