Related Clues:

• Watch-crystal mineral
• Mineral tinged with impurities giving a variety of shades
• Hard mineral such as amethyst or citrine, say
• Hard mineral consisting of silicon dioxide in crystal form
• Finely ground quartz
• Common rock mineral
• Amethyst is a purple form of it
• Colourless glass made of almost pure silica
• Hard mineral composed of silicon dioxide
• Hard mineral such as amethyst or citrine