How do I identify Cordierite?

Cordierite can be identified by its hardness of 7 - 7.5 on the Mohs scale, Grey color. Look for these key characteristics when examining specimens.

What color is Cordierite?

Cordierite typically appears in Grey, blue, blue-violet, greenish, yellowish brown, colourless to very light blue. Color can vary depending on impurities and formation conditions.

How hard is Cordierite?

Cordierite has a hardness of 7 - 7.5 on the Mohs scale. This makes it a relatively hard mineral suitable for jewelry.

Cordierite

A variety of Minerals

What is Cordierite?

Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: (Mg,Fe)2Al3(Si5AlO18) to (Fe,Mg)2Al3(Si5AlO18). A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings.

Optical Properties

Refractive Index: 1.542-1.578
Birefringence: 0.008-0.012
Pleochroism: Strong trichroism: colorless to yellowish, pale blue, dark (violet)-blue
Optical Character: Biaxial positive or negative
Dispersion: 0.017

Cordierite Market Value Calculator

Estimate the market value of Cordierite using size, quality, and finish. This preview calculator is for quick context and is not a formal appraisal.

Cordierite Localities Map

See where Cordierite is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.

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North ZoneCentral RidgeSouth Basin

The Meaning

Cordierite was used by the Vikings to find the direction of the sun on overcast or rainy days and is mentioned in "The Saga of King Olaf". Symbolically, cordierite is the gemstone that is associated with a 21st wedding anniversary.

Key Characteristics

Formation of Cordierite

Cordierite typically occurs in contact or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs, for example, in the granite contact zone at Geevor Tin Mine in Cornwall.

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Quick Facts

Physical Properties

Color: Grey, blue, blue-violet, greenish, yellowish brown, colourless to very light blue
Hardness (Mohs): 7 - 7.5
Density: 2.505 g/cm³
Streak: White