Cordierite Value, Price, and Jewelry Information

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CORDIERITE: Iolite, India (1.56, 8.51, 3.00). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Cordierite

The crystal structure of cordierite has many similarities to that of beryl; indialite, the dimorph, in fact has the same structure as beryl. Iolite with hematite inclusions (bloodshot iolite) comes from Sri Lanka. The inclusions sometimes yield a gem showing a 4-rayed star (quite rare). The blue color of iolite along one optical direction strongly resembles sapphire, and such gems, correctly oriented in settings, are often confused with sapphires. Iolite is not a very rare material, but stones that are completely clean over 10 carats are quite uncommon, and clean 15-20 carat gems are worthy of museum display.

Cordierite Information

Data	Value
Name	Cordierite
Varieties	Iolite
Colors	Blue, bluish violet, smoky blue; rarely greenish, gray, yellowish, brown.
Crystallography	Orthorhombic. Crystals prismatic with rectangular cross section; also massive, granular; may be pseudohexagonal. Note: Indialite is hexagonal.
Refractive Index	1.522-1.578
Luster	Vitreous.
Hardness	7 - 7.5
Fracture	Conchoidal
Specific Gravity	2.53-2.78. Most gems are 2.57-2.61 (higher with higher Fe content).
Birefringence	0.005-0.018
Cleavage	Distinct 1 direction
Dispersion	0.017
Stone Sizes	Iolites are frequently in the 1-10 carat range, dichroic with blue to violet color. Large clean stones free of inclusions are not common at all, but gems over 30 carats have been reported British Museum (Natural History) (London England): worked crystal fragment of 177 grams. National Museums of Canada (Ottawa, Ontario): 2.20, 3.93, 2.60 (Canadian localities). Private Collection: 17. Smithsonian Institution (Washington, D.C): 15.6, 9.4 (blue, Sri Lankan); 10.2 (indigo, Sri Lanka).
Luminescence	None (quenched by Fe).
Luminescence Present	No
Transparency	Translucent to transparent.
Absorption Spectrum	Iron spectrum. Weak bands at 6451, 5930, 5850, 5350, 4920, 4560, 4360, and 4260. Spectrum observed varies with direction of crystal.
Phenomena	Asterism (very rare).
Formula	(Mg,Fe)₂ Al₄Si₅O₁₈
Pleochroism	Intense and distinctive. Fe-rich crystals: a = colorless; γ = violet. Mg-rich crystals: pale yellow to green/pale blue/violet, violet-blue.
Optics	a = 1.522-1.558; β = 1.524-1.574; γ = 1.527-1.578. (Sri Lanka: a = 1.530; β= 1.534; γ = 1.539; Birefringence = 0.009; S.G. = 2.58). Biaxial (+), 2V= 65-104°. Frequently optically (-).
Optic Sign	Biaxial +, Biaxial -

Gem names: lolite; Water Sapphire. Dimorph of Indialite.

Optics: a = 1.522-1.558; β = 1.524-1.574; γ= 1.527-1.578. (Sri Lanka: a = 1.530; β= 1.534; γ = 1.539; Birefringence = 0.009; S.G. = 2.58). Biaxial (+), 2V= 65-104°. Frequently optically (-).

Inclusions: Crystals of apatite and zircon, the latter with pleochroic haloes, the outer edges of which are deep yellow. Frequently dustlike masses of tiny crystals. Also hematite plates in parallel orientation (from Sri Lanka) impart a red color, and gems are sometimes called bloodshot iolite.

Occurrence: In altered aluminous rocks; igneous rocks; alluvial gravels.

California; Idaho; Wyoming; South Dakota: New York; New Hampshire.

Great Slave Lake. Canada: Greenland: Scotland; England; Norway, Germany; Finland.

Connecticut: gemmy material that cuts up to 2 carats.

Madras, India: gem iolite in abundance.

Sri Lanka and Myanmar: gemmy material from the gem gravels.

Paraiba, Brazil: some gemmy material from Virgolandia, in nodules.

Babati, Tanzania: gem material.

Karasburg, Namibia: gem material. Madagascar; Japan: Australia.

Comments: The crystal structure of cordierite has many similarities to that of beryl; indialite, the dimorph, in fact has the same structure as beryl. Iolite with hematite inclusions (bloodshot iolite) comes from Sri Lanka. The inclusions sometimes yield a gem showing a 4-rayed star (quite rare). The blue color of iolite along one optical direction strongly resembles sapphire, and such gems, correctly oriented in settings, are often confused with sapphires. Iolite is not a very rare material, but stones that are completely clean over 10 carats are quite uncommon, and clean 15-20 carat gems are worthy of museum display.

Name: After Mr. Cordier, a French geologist who first studied its crystals. Iolite is from Greek ios (Violet) and lithos (stone).

by Joel E. Arem, Ph.D., FGA