Hin sila kohn are a kind of [imitation or manipulated] rocks that purportedly are found in river beds and caves along the Mekhong River area and which contain [artificial or low value] gemstones that are associated with the mythical naga and known as phet phayanaag (fig.).  In order to get to the naga-gems the rocks need to be broken open with a hammer.

These gemstones, seen in the square container in the top left corner, are not embedded in the rocks but usually sit rather loosely in a cavity inside the rocks, seen on the crushed rock or brick in the bottom right corner.