Dioptase is a Copper Silicate most well known as being rhombohedral, vibrantly emerald green crystals. Crystals of Dioptase exceeding 0.75cm are considered rare which is largely believed to be linked to its complexity of formation.
Dioptase is in the Trigonal crystal system and while found in a large variety of locales, the specimens from Tsumeb Mine are considered to be the best. However, this can be considered largely opinion as many locations show formations and associations that are not readily available from Tsumeb.
What to look for: Isolation and Formation of Crystals, Size of Crystals, Associations, Luster, Specimens on Matrix, and, Unusual Localities.