Cordierite

Characteristics

• Habit - prismatic
• Form - orthorhombic
• Hardness - 7 - 7.5 (not very tough)
• Cleavage - moderate to poor
• Fracture - conchoidal to uneven
• Streak - white
• Lustre - vitreous to greasy
• Colour - blue, violet-blue, grey, blue-green

Found in high-grade thermally metamorphosed alumina-rich rocks. Also found in gneisses and schists, maybe in granites and veins of quartz. Important in the production of ceramics used in catalytic converters in cars.

Iolite

Blue. Also known as 'water-sapphire'. Best blue colour is seen on the length of the prismatic crystal.

White light passes through easily in one direction and appears almost colourless but in other directions is absorbed and the stone looks blue, like sapphire. (pleochroism)

Known for trichroism which can usually be seen with the eye by turning the stone. Under a dichroscope can see yellow / blue / violet.

Sources in Sri Lanka, India, Madagascar, Brazil, Tanzania, Norway.

Lore: believed that Vikings used it as navigation aid by finding the direction of the sun.