Ilvaite is a sorosilicate and calcium and magnesium substitute in the structure. Ilvaite crystallizes in the monoclinic system in black prismatic crystals and columnar masses. It is black to brownish black to gray and opaque. It has a Mohs hardness of 3.8 to 4.1. Ilvaite is structurally related to lawsonite.
It occurs in contact metamorphic rocks and skarn.
Ilvaite was first described in 1811 on the island of Elba and the name ilvaite from the Latin name ilva of the island. Sometimes referred to as yenite.