Topaz is thought to form from fluorine bearing gases given off during the later stages of solidification in acidic igneous rocks, occuring in cavities and fissures within rhyolites and granites, and also in granitic pegmatites where it is commonly associated with quartz, microcline, muscovite,tourmaline & beryl. Other minerals sometimes associated with topaz in pegmatites are spodumene, lepedolite and apatite. Topaz occurs in metamorphic rocks by fluorine metasomatism in greisens and quartzites associated with fluorite, zinnwaldite, corundum and rutile. It also occurs (rarely) in schists. Topaz appears in hypothermal ore deposits associated with cassiterite, hematite, wolframite and gold. Due to topaz's hardness, it is often found in eluvial and alluvial deposits. Topaz is characterized by its high specific gravity, hardness, crystal habit and cleavage.