20	PRECIOUS STONES.
	owe their origin to the causes tabulated under B5 and B6. We find it developed as a secondary mineral in highly-silicated rocks which have been subjected to dynamic changes ; the beginning of its formation only commenced after many other changes had occurred in the rock, since its crystals are attached to crystals of pre-formed minerals, but its growth was completed before the cessation of the movements which caused the metamorphosis; the last movements fractured the Tourmaline crystals, chiefly transversely to their axis of greater elongation, and bent them, and then the fissures so formed were filled with the last of the rock-forming minerals to consolidate, since Quartz, Felspar and Mica are found in these interstices. The absorption of the ends of the crystals seems to have occurred at the time of the bending; it is comparatively rarely one finds well terminated crystals, though specimens showing the prism zones well developed and perfect are common.