This specimen is massive barite from strongly metamorphosed dolomite marble in the Gavilan Range of California. Barium-bearing solutions entered the stone during this metamorphism, but conditions did not favor good crystals. The weight alone is the diagnostic feature of barite: its hardness is 3 to 3.5, it does not respond to acid, and it has right-angled (orthorhombic) crystals.
Barite is widely used in the drilling industry as a dense slurrydrilling mudthat supports the weight of the drill string. It also has medical uses as a filling for body cavities that is opaque to x-rays. The name means "heavy stone" and it's also known by miners as cawk or heavy spar.