Kyanite is an aluminium silicate that grows in long, blade-like crystals, and its best material is a rich sapphire blue. The name comes from the Greek kyanos, meaning deep blue. Its famous quirk is directional hardness: around 4.5 to 5 along the length of a blade, but 6.5 to 7 across it. That made it a nightmare for early cutters and earned it the old name disthene, meaning two strengths. The finest blue kyanite comes from Nepal, with more from Brazil, India, Tanzania and the USA.
Because of that split personality, kyanite is a stone for gentle wear. Its perfect cleavage means a knock along the wrong direction can split it, so it belongs in pendants and earrings rather than everyday rings. It is faceted when clean enough, and cut as cabochons when it shows its attractive streaky structure. Skip ultrasonic cleaners entirely and wash it in lukewarm, soapy water. The reward for the extra care is a natural, untreated blue that genuinely rivals sapphire.
Fellow violet-blue gems: iolite and tanzanite are its main colour rivals, and dumortierite matches its inky tone in opaque form.
