Page:A treatise on diamonds and precious stones including their history Natural and commercial.djvu/172

 bing down the stones, is a horizontal plate of soft lead ; its surface is supplied with wet emery powder, which becomes indented in it by the pressure of the stone.

a stone is to be polished, the lead mill is removed and replaced by one of pewter,which is first prepared by what is termed hacking. This operation is performed by holding the edge of a knife perpendicularly upon it, and turning the mill in contrary directions, until a rough surface is produced. Rotten-stone is then applied to it, with a little water, and the stone is held on the mill until it has received the polish required.