http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
Its
reflecting power and refractive energy, on
the other hand, exceed those of all other
colourless bodies, while it yields to none in
the perfection of its pellucidity.” He was
constrained to add, “The formation of the
diamond is an unsolved problem.”