Kyanite, Hard One Way and Soft the Other

Kyanite is an aluminium silicate, Al2SiO5, and its most distinctive property is that its hardness depends on direction: a crystal is about 4.5 along its length and 6.5 to 7 across it, so it can be scratched by a knife in one orientation and not in the perpendicular one. This unusual anisotropy is why the mineral was once called disthene, two strengths. It forms as bladed, often sky-blue crystals, the colour giving the name from Greek kyanos, dark blue, though it also occurs green, black and white.

Kyanite is important to geologists as an index mineral: it, andalusite and sillimanite share the same chemical formula but form under different pressures and temperatures, so which one is present records the conditions a rock experienced. It carries no historical gem tradition; its appearance in modern crystal practice, often framed around alignment and communication, is recent and colour-led.