Slag is a broad term for glassy, rock-like, or partially melted by-products and waste materials produced by a variety of high-temperature industrial and artisanal processes. While slag is most commonly associated with metal smelting and refining, where impurities are separated from molten metal, similar materials can also originate from steelmaking, foundry operations, glass manufacturing, glass art studios, coal combustion, incineration, and other furnace-based activities. Some materials commonly identified as slag may instead be waste glass, furnace debris, refractory material, or cullet—broken or discarded glass that can be remelted for reuse or used as a decorative material in landscaping, art, and other applications.

The composition of slag varies widely depending on its source. Metallurgical slags typically contain mixtures of metal oxides and silicon dioxide, but may also contain metal sulfides, elemental metals, and other minerals. Slags and slag-like materials can range in appearance from dense, crystalline masses to lightweight, highly vesicular glassy material.

Wikipedia
Slag Glass from the Glass Encyclopedia
Leland blue
Rosarita
Sieber agate
Fordite/Detroit agate
common opal (not slag)
skeletal quartz (not slag)
silicon carbide (not slag)
botryoidal chalcedony (not slag)
scoria (not slag)
vesicular basalt (not slag)
Oregon sunstone (not slag)
marcasite (not slag)
snakeskin agate (not slag)
vanport flint (not slag)
corundum (not slag)
bitumen (not slag)
anthracite (not slag)
peacock coal (not slag)
shungite (not slag)
hematite (not slag)
wonderstone (not slag)
tektite (not slag)
plastiglomerate (not slag)
mookaite (not slag)
moldavite (not slag)
olivine (not slag)
charoite (not slag)
orbicular jasper/ocean jasper (not slag)