Benstonite is a rare, barium-calcium-magnesium carbonate mineral, Ba6Ca6Mg(CO3)13. Chemical studies show that strontium and manganese often substitute for the barium and calcium respectively. Benstonite has a nonmetallic, glassy luster, a white to off-white to pale yellow to yellowish-brown color, a white streak, a hardness of 3 to 4, and has good cleavage. It forms translucent, rhombohedral crystals. The best benstonite specimens on Earth are from Illinois' Cave-in-Rock Mining District.
Benstonite from the Bethel Level, Minerva # 1 Mine, Cave-in-Rock Mining District, southern Illinois, USA.