Cordierite
Cordierite is a silicate mineral prized by collectors for its exceptional color range, with notable Chinese occurrences.
Cordierite is a magnesium-aluminium silicate whose transparent blue-violet gem variety is known as iolite — famous for showing different colours from different directions.
About Cordieriteextended article
Cordierite is a magnesium-aluminium silicate best known to gem lovers as iolite, its transparent blue-to-violet variety. Its most striking feature is strong pleochroism: the same crystal can look blue, grey or nearly colourless depending on the viewing direction.
Identifying cordierite
Cordierite is fairly hard (7–7.5) with a glassy lustre, often grey or blue and frequently included. The dramatic colour change with direction (pleochroism) of gem iolite is diagnostic and earned it the nickname 'water sapphire'. It forms in aluminium-rich metamorphic rocks.
Where it is found
Gem cordierite (iolite) comes from Sri Lanka, India, Madagascar, Myanmar and Tanzania, among others, weathering out into gem gravels. Massive cordierite occurs in metamorphic rocks worldwide.
About Cordierite
Cordierite belongs to the silicate class in the cordierite group / indialite group and has the chemical formula (Mg,Fe)2Al3(AlSi5)O18. It crystallizes in the hexagonal system and is one of the most visually varied minerals in the collector market.
Identification & care
Crystals commonly develop as short prismatic crystals (pseudo-hexagonal appearance); also massive, granular. Its color range is broad, including blue, violet-blue, pale blue, gray, pale yellow, and pale brown. The luster is vitreous, the streak is white, and specimens range from transparent to translucent. The cleavage is distinct {100} and {001}. The fracture is subconchoidal to uneven, which aids identification.
Collector context
Collector notes
For collectors, Cordierite is a benchmark crystalline species. Documented Chinese occurrences are recorded at Yaogangxian W-Sn ore field and Shizhuyuan Mine, among others.
Frequently asked questions
What is Cordierite?
Cordierite is a silicate mineral prized by collectors for its exceptional color range, with notable Chinese occurrences.
What is the chemical formula of Cordierite?
The chemical formula of Cordierite is Mg2Al4Si5O18.
What crystal system does Cordierite belong to?
Cordierite crystallises in the Orthorhombic crystal system.
References & databases
Mindat.org is the world’s largest open mineralogy database. Our descriptions are written independently and fact-checked.