Amethyst is a purple form of it (6)

I believe the answer is:

quartz

(Other definitions for quartz that I've seen before include "Glassy mineral" , "Stone such as amethyst or cairngorm" , "Colourless mineral used to make glass" , "Hard mineral found in watches" , "Hard glossy mineral found in rocks" .)

I've seen this clue in the USA Today.

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