Blue topaz
Variety Al2SiO4(F,OH)2
Also called Swiss blue topaz and London blue topaz
About it
Nearly all blue topaz is colourless topaz irradiated and heated to blue; natural blue is pale. Texas's state gem is blue topaz from Mason County; the deep "London blue" shade is a treatment.
- Type
- Variety
- Group
- Topaz variety
- Formula
- Al2SiO4(F,OH)2
- Hardness (Mohs)
- 8
- Specific gravity
- 3.5–3.57
- Streak
- white
- Lustre
- vitreous (glassy)
- Crystal system
- Orthorhombic
- Colours
- sky blue, Swiss blue, London blue
- Transparency
- transparent
- Cleavage and fracture
- perfect basal in one direction; conchoidal fracture
Typical values: real specimens vary. For more, see mindat.org.