Black Kyanite is in the Triclinic Crystal System in the class of Silicates and forms in fibrous and twisted, or crossed, long crystals. The black variety can appear as a dark gray to dark black. It is found in South America, Africa, Scandinavian regions, and Switzerland as well as here in the USA in California and New Hampshire. It is most frequently found in igneous stone like many of our gem friends are.
Black Kyanite can be cracked fairly easily, but it depends on which way you orient it. It is harder across its grain than along it. Long pieces can be fairly easily separated, but trying to crack a long piece in half is much harder to do (for my geology friends, this attribute is called anisotropic). In MOHs hardens this means it’s a 6.5 across the crystal but only a 4.5 along the length parts. Most pieces used in jewelry have many crossed strands making them solid to wear without the worry of them snapping.
I should also mention that the Black, like the other Kyanite varieties and Citrine, is also self clearing. It does not hold negativity so it can be used repeatedly without needing clearing or recharging. This makes it an awesome one to use in our healing work.