Chemical composition -- A complex mix of metals, oxygen, hydrogen, and fluoride.
Color-- Various shades of green, yellow, brown.
Optics -- R.I. 1.66-1.76.
Durability -- Hardness 6-7.
Crystal structure -- Tetragonal.
Specific Gravity -- 3.32-3.47.
Sources -- Italy and Quebec, Canada, are well known locales for yellow, green, and brown idocrase up to about 10 or 15 carats. Faceted stones often resemble grossular garnet. While the material is not particularly rare, faceted stones are not often seen.