Iolite
Variety Mg2Al4Si5O18
Also called Water sapphire and Dichroite
About it
Gem cordierite that shifts from violet-blue to nearly clear or yellowish grey as it turns (strong pleochroism). India, Sri Lanka, and Madagascar supply most.
- Type
- Variety
- Group
- Cordierite variety
- Formula
- Mg2Al4Si5O18
- Hardness (Mohs)
- 7–7.5
- Specific gravity
- 2.56–2.66
- Streak
- white
- Lustre
- vitreous (glassy)
- Crystal system
- Orthorhombic
- Colours
- violet-blue, blue, greyish blue
- Transparency
- transparent, translucent
- Cleavage and fracture
- poor; conchoidal fracture
Typical values: real specimens vary. For more, see mindat.org.