It is a piece of industrial slag, in this case waste from smelting operations, probably iron smelting.
Industrial slags take a wide range of forms: frothy, lumpy, glassy... It depends on the chemistry and physics in play. Yours is glass. Ordinarily, glass fractures with a conchoidal pattern, which we don't see here. But that's because the (fresher) fracture surface is actually along a chemical boundary. The layers you see as differently coloured swirls in the older surface are layers of slightly different chemistry. Such layers are usually very well stuck together, but in this case one of them has provided a surface along which the slag split (You don't see the swirling on the broken surface because none of it crosses that surface.) This layer appears to have had a high metal content, giving the broken surface a somewhat metallic lustre. The most recent break is shown by the brightest patch in the middle of your photo. Above that, a slightly older surface is oxidizing to a dark purplish colour.
These turquoisy slag glasses are often associated with iron smelting. A pale blue colour may indicate iron (Fe2+), greener shades may indicate copper (which can also be blue). Remember, the slag contains impurities from the smelting process, so iron smelting slag may well contain metals other than iron.
I suspect there is/was iron (maybe copper) mining and/or smelting upstream or nearby.