A popular mineral among collectors who has been recently re-considered : check
Is a Hydrated Potassium Calcium Sodium Silicate Fluoride Hydroxide with formula
(K,Na)Ca4Si8O20(F,OH) - 8H2O .
Although is quite common in natural crystal, is very difficult to cut and polish, due to his easy cleavage: a challenge for the cutters.
The hardness is 4.5-5 and this gem here comes from a classic locality: Poona,India.
Eye clean, cut by Luigi Marian, a top quality stone!