A polished handmade marble no longer has pontils. A polished machine made marble is missing the top layer of glass. A machine made marble that still retains some of its original surface is also referred to by the same name. SLAG - a marble made from two different colors of glass that were melted together in the same furnace pot. Due to the differing densities of the glass, they would not melt into a homogeneous color.
Handmade slags have pontils.
Machine made slags consist of a colored transparent glass with opaque white swirls. No obvious moon under the surface. The spot just sparkles in the light. No glass is missing. Circular bruise mark.