A mixed oxide mineral of beryllium
and aluminum (BeAl2O4).
It is found in beryllium-rich pegmatite
dikes; Brazil is a leading source. Its hard,
hexagonal system crystals are prismatic or tabular. It can be transparent,
green, yellow, or brown, but bright yellow-green is most highly valued.
Alexandrite and cat's eye are gem
varieties. Hardness 8.5, relative density 3.69.
AND PLANETARY SCIENCE