iolite

Iolite is the gem-quality variety of the mineral cordierite, an aluminum silicate mineral that forms in metamorphic rocks. It is best known for its strong pleochroism, typically appearing blue to violet when viewed in certain orientations and colorless or pale brown along another axis. This characteristic makes iolite a distinctive gemstone, as cutters orient the crystal to maximize the preferred blue-violet hue.

Cordierite, and by extension iolite, forms in metamorphic environments, particularly under low- to medium-grade conditions. It is commonly associated with minerals such as sillimanite, quartz, biotite, and garnet and occurs in rocks like granulites and schists. The mineral typically exhibits a vitreous luster and can appear as elongated crystals; on the Mohs scale it generally ranges around hardness 7, with a specific gravity near 2.6.

Iolite is the trade name for gem-quality cordierite. It is mined in several countries, with notable sources including Finland (Lapland), Madagascar, Sri Lanka, and India. Because of its pleochroism, gem cutters faceting iolite aim to bring out the vivid blue-to-violet color in the face-up orientation.

Cultural notes include a longstanding, though debated, association with Viking navigation. Some folklore claims that Vikings used transparent iolite to locate the sun on cloudy days, functioning as a sun compass; however, evidence for this practice is not conclusive and Iceland spar (calcite) is also discussed in similar contexts.