Atrikite

Atrikite, known more commonly as blade crystal, is a naturally-occurring mundane crystal that forms with a flat rhombus shape on its horizontal cross-section, like a double-edged sword.

This crystal commonly grows where you can find other crystals, most often on the surface, and can present a serious danger to travelers that don't watch their step. Falling into an atrakite outcropping can easily be fatal.

Pure, unspoiled atrikite has an opaque white coloration, but it tends to take in small amounts of the elements of other crystals nearby as it grows and tints toward their coloration, making it harder to differentiate between them at a glance and thus makes them yet more dangerous to travelers. However, those impurities from absorbing other elements can give some atrikite crystals special properties depending on the element absorbed.

For obvious reasons, this crystal is collected and used widely as common knives. However, the crystal is quite brittle so it can't be used in its raw form for cutting harder or tougher materials, and trying to use a raw atrikite sword in combat will most likely end with your sword being shattered. Several methods exist for making the crystal more durable, most involving heating the crystal to near its melting point and treating it with carbon or other powdered crystals suspended in a mixture. Atrikite weapons treated this way can be as durable as steel, but keep an edge much, much better than steel.