264	PRECIOUS STONES.
	amadou or German tinder, the whole bound with wire and the tinder lighted and allowed to smoulder away; if the heating is too great the stone will be completely decolorised. It is remarkable that a "burnt" Topaz shows more marked dichroism than the unaltered mineral, images of a cherry
	red and a pale yellow being seen in the dichroscope. The fact that Topaz may be so strongly heated without destroy­ing the colour points to the colour not being due to organic matter; but in some Topaz organic matter does seem to account for the colour. Certain specimens of yellow and blue Topaz are found to change their tints on exposure to strong light, for instance. Topaz becomes strongly