http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
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. FIG. 3. KIMBERLEY MINE. THE “PIPE.” FIG. 4. SECTION OF KIMBERLEY MINE.