106	PRECIOUS STONES
	and moisture of their surroundings ; alumina, as well as the oxides of copper, manganese -and iron, are often washed across and over these bones lying on the cave floor, so that in time, this silt acts on the substance of the bones, forming a variety of turquoise of exactly the same composition as that just described, and of the same colour. So that around the bones there eventually ap­pears a beautiful turquoise casing ; the bone centre is also coloured like its casing, though not entirely losing its bony characteristics, so that it really forms a kind of ossified turquoise, surrounded by real turquoise, and this is called the " bone turquoise " or " odontolite"