Topaz

Two end members:

• hydroxy-topaz: Al₂SiO₄(OH)₂
• fluorine-topaz: Al₂SiO₄F₂ – (Maybe some fluorine could be replaced with the more common chlorine?)

A pretty hard aluminum silicate.
In terms of elemental composition between sapphire and quartz.

Other properties:

• Mohs 8 (defining mineral)
• Orthorhombic – (Nesosilicate like olivine)

External links

• Wikipedia: Topaz
• on www.mineralienatlas.de
• Wikipedia: Mohs scale