http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
They are irregularly shaped round or oval
pipes, extending vertically downwards to
an unknown depth, retaining about the
same diameter throughout ( Fig. 3 ). They
are said to be volcanic necks, filled from
below with a heterogeneous mixture of fragments
of the surrounding rocks, and of older
rocks such as granite, mingled and cemented
with a bluish-coloured, hard clayey mass,
in which famous blue clay the imbedded
diamonds are hidden.