BENITOITE
Benitoite is a very rare barium titanosilicate mineral
(BaTiSi3O9). It is valued as a gemstone and as a
mineral specimen. Faceted and polished benitoite gemstones have a
wonderful deep blue color and approximate diamond's index of refraction (the
"fire" of diamond). Benitoite occurs only in certain
metamorphic environments having sufficient barium and titanium (a rare
combination).
Benitoite has a nonmetallic, slightly glassy luster, a
blue color (typically), a white streak, has a hardness of 6 to 6.5, essentially
no cleavage, conchoidal fracture, forms truncated double triangular pyramid
crystals, is translucent to transparent, and fluoresces bright blue under black
light (UV light).
Benitoite (crystal is 8 mm across at its widest) on blueschist (glaucophanite)
from the Dallas Gem Mine, San Benito County, California, USA.
Benitoite on blueschist (glaucophanite) from the Dallas Gem Mine, San Benito
County, California, USA. (public display, Field Museum of Natural
History, Chicago, Illinois, USA)