Also known as cordierite or water sapphire

Physical properties:

Chemistry: Mg2Al3[AlSi5O18]
Hardness: 7–7½
SG: 2.58–2.66
Cleavage: Imperfect, three directions
Fracture: Somewhat brittle
Heat sensitivity: Low

Optical properties:

RI: 1.53–1.55
Dispersion: 0.017 (medium)
Double refraction: 0.012 (weak)
Pleochroism: Very strong
Critical angle: 40°

Facet angles (pavilion/crown):

Vargas: 43°/42°
Soukup: 43°/42°
Roth: 43°/42°
MDR: 43°/42°
Schlagel: 42°/38°
Sinkankas: 43°/40-50°
GIA: 43°/42°
Perkins: 42°/38°
Cornwall: 42°/38°
Weikoff: 43°/39°
Carroll: 43°/35.3°

Cutting lap:

Any grit size

Facet polishing:

Vargas: Cerium oxide on acrylic, Aluminum oxide on tin/lead
Christiansen: Aluminum oxide on tin/lead, Tin oxide on tin
Perkins: Aluminum oxide on BATT or Corian

Cabochon polishing:

Christiansen: Aluminum oxide or diamond 50k, 100k on leather or wood, Tin oxide on wood


Orient carefully to achieve fine blue/violet color. Cleavage planes rarely pose any problems.


None known

Iolite rough

Iolite on Gemdat

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