Iolite is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of magnesium aluminum silicate, their full chemical compound being Mg2Al4Si5O18.
Iolite is the violet-colored variety of Cordierite (named after French geologist P. Cordier), a silicate of aluminum and magnesium, with an orthorhombic crystal system. Though the name cordierite is used by mineralogists, the name iolite has become established among gemologists.
It appears as stubby, prismatic crystals, pesudo-hexagonal twins with a glassy appearance. It is frequently microgranular and massive.
It is very hard, light, has poor cleavage and conchoidal fracture, and is usually translucent with vitreous luster. It displays pleochroism visible to the naked eye, appearing blue or violet when viewed parallel to the prism base, colorless when viewed vertically. Care must thus be taken when cutting.
Some iolite shows aventurine effects caused by flaky inclusions of hematite, or asterism owing to silky inclusions.
It can be distinguished from quartz because it is fusible on thin edges and is twinned differently. It is softer than corundum and insoluble in acids.
It occurs widespread in hornfels produced from argillaceous rocks, as oval bluish spots or cavities; also in gneisses and schists where pressure was low. It may be found in norite or granite, where clay has been incorporated. It is associated with garnet, mica, quartz, andalusite, sillimanite, staurolite, chalcopyrite, and spinel.
Small, round transparent pebbles, attractively colored, are found in sand and gravel in Sri Lanka and Madagascar. Deposits are found in Scotland, Sweden, Canada, USA. Some good crystals come from Thompson, Manitoba, Canada. The gem gravels of Myanmar also produce iolite. Blue crystals (saphire d'eau) from Sri Lanka are used as gemstones.
It is also called cordierite, dichroite, and often misnomered water sapphire (another variety of cordierite).
Iolite derives its name from its violet color.
Bluish-white specimens of cordierite were probably used by the Vikings as navigational aids. They could tell the points of the compass even when the sky was overcast, because, depending on the way it was viewed, the mineral changed color in polarized light (pleochroism). For this reason, it has also been dubbed, "Vikings' compass."
Gem-quality iolite is attractive and readily available, though not considered valuable and is not widely used, but when transparent, it is faceted as a gem, and is of interest to scientists and collectors. Semi-transparent gems are cut cabochon.
Composite grains of iolite and sunstone India are used to make single stones with coupled deep blue and orange-brown hues. Most gem iolite occurs as water-worn pebbles.
Iolite is fashioned so that the blue colour is uppermost on a faceted stone or in the front of a statuette. The pleochroism is strong with violet-blue, pale blue and pale yellow to green colours.