Barite is a barium sulphate mineral, relatively common. It also quite often forms good crystals which are usually tabular or platy in habit. This one is from the rocky southern coastline of Phillip Island in Victoria, one of my favourite collecting places when I lived on the mainland. Its now off-limits to collectors (some sort of marine reserve?). Barite occurs here with zeolite minerals in cavities (amygdules) in the basalt rock.
It is also one of the specimens coming up in a lot in this week's auction.