Oklahoma designated hourglass selenite crystal as the official state crystal in 2005. These crystals are found only on the salt plains of Oklahoma, a unique 11,000-acre geological area. The salt plains were formed by repeated flooding by sea water millions of years ago.

Selenite (hydrous calcium sulfate) is a crystallized form of gypsum, a common mineral in sedimentary environments that can take on a great variety of crystal forms and shapes. Crystals take on the characteristics of their environment - iron oxide in the soil gives the hourglass crystals their chocolate brown color.

On the salt plains, sand and clay particles often form an "hourglass" shape inside the crystal. Crystals measuring up to 7 inches long have been found, along with complex combinations weighing as much as 38 pounds.

Source:

Selinite Crystals: USFWS Salt Plains Nat. Wildlife Refuge

Links:

Geological Symbols - 50 States List