Monday, 28th. Passed an immense quarry of beautiful white alabaster; 3 buffalo killed, distance traveled 18 miles, encamped on a little rivulet affording as good water as ever run.

On the same day, James John wrote in his journal:

Took dinner on the bank of a creek of good water and good grass. Near there is a large cliff was white as snow, a little harder than common chalk and some of the earth is composed of a substance, supposed by the Company to be plaster of Paris.

Alabaster is a form of gypsum and is indeed used in making plaster of Paris.

Father De Smet wrote about the alabaster quarry.

We discovered a curious quarry, which, at first, we took for white marble, but we soon found it something more valuable. Astonished at the facility with which we could fashion this kind of stone into any shape, most of the travellers made calumets of it. I had several made myself, with the intention of offering them as presents to the Indians, so that for the space of forty-eight hours our camp was filled with lapidaries. But the great number of these calumets could not withstand the action of the fire, and broke. It was alabaster.

The dictionary definition of alabaster at Lexico.com says it is a “fine-grained, translucent form of gypsum, typically white, often carved into ornaments.” Its relative softness makes it easy to carve.

A calumet is a ceremonical pipe used by Native Americans. Here’s a picture of one carved out of salmon alabaster.