http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
The thickness of
yellow earth in the mines is therefore a
measure of the depth of penetration of air
and moisture. The colour does not affect
the yield of diamonds. Besides diamonds, there have been detected
more than eighty species of minerals
in the blue ground, the more common
being magnetite, ilmenite, garnet, bright
green ferriferous enstatite (bronzite), a
hornblendic mineral closely resembling
smaragdite, calc-spar, vermiculite, diallage,
jeffreysite, mica, kyanite, augite,...