Tis forTopaz

Pure topaz is made from crystalline silicate of aluminium and fluorine. When it is pure it has no colour, but can be coloured pale yellow, brown, pale green, reddish or even rare instances blue if the stone contains impurities.
Topaz has been used in jewellery for thousands of years, and originally any clear yellow stone was called topaz. Now only stones that are the correct chemical composition can be called topaz.