Name Iolite
Chemical Composition Mg2Al4Si5O18
Chemical Composition (Alternate) (Mg,Fe)4Al8(OH)2(Si2O7)5
Hardness 7.0 to 7.5
Refractive Index 1.53 - 1.54 to 1.54 - 1.55
Bi-refringence 0.001
Optic Sign Negative
Optical Character Biaxial
Specific Gravity 2.58 to 2.66
Crystalline System Rhombic
Only above 1 Kilo Lots of all size
