http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
The diamantiferous breccia filling the
mines, usually called “blue ground,” is a
collection of fragments of shale, various
eruptive rocks, boulders, and crystals of
many kinds of minerals. Indeed, a more
heterogeneous mixture can hardly be found
anywhere else on this globe.