triangle — a shape in faceting a gemstone …
tridymite — species of mineral; data; constituent of opal …
triphylite — species of mineral; data; faceted specimens exist …
triplite — species of mineral; see triplite in ‘Classification’ …
tsavorite — variety of grossular; see tsavorite in ‘Classification’ …
tuff — a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption …
turquoise — species of mineral; see turquoise in ‘Classification’ …
ultramafic rock — igneous and meta-igneous rock with a very low silica content (less than 45%), composed of usually greater than 90% mafic minerals …
umbalite — a synonym of rhodolite; mined in Umba Valley, Tanzania …
uvite — species of mineral; see uvite in ‘Classification’ …
variscite — species of mineral; see variscite in ‘Classification’ …
väyrynenite — species of mineral; see väyrynenite in ‘Classification’ …
verdelite — variety of elbaite; see verdelite in ‘Classification’ …
vesuvianite — species of mineral; see vesuvianite in ‘Classification’ …
vlasovite — species of mineral; see vlasovite in ‘Classification’ …
volcanic pipe — usually largely composed of one of two characteristic rock types — kimberlite or lamproite; they are well known as the primary source of diamonds …
watermelon tourmaline — variety of elbaite; see watermelon tourmaline in ‘Classification’ …
whistle — rare shape in faceting a gemstone …
willemite — species of mineral; data; faceted specimens exist …
window — rare shape in faceting a gemstone …
