Glassy and vesicular = Slag
Many of the meteorwrong photos sent to me are of pieces of slag. Slag usually refers to a glassy by-product of smelting ores to retrieve the metal. I use the term here to also include any man-made, rock-like by-product of heating things to high temperature. Thus, clinkers are a form of slag. Slags catch people attention because of their morphology. Also, many slags contain metal from inefficient separation of metal from the ore and, thus, they will attract a magnet. Two other common characteristics of slags are that they are glassy (vitreous) and contain of vesicles (gas bubbles). Slags often have very rough exteriors, unlike any stony meteorite. Some show flow features in the glass and others have flat surfaces from having solidified in a contained space. Slags are sometimes used in road construction and as landscaping gravel, so they are more common than one might think.
I have not personally examined all the examples pictured here, but I suspect that most are slags. From the stories that accompanied some of these photos, the “rocks” may be products of military explosions. Others might be fulgurites – lithified soil and vaporized organic matter produced as a result of a lightning strike. Some might be products of electric discharges when high-voltage power lines fall to the ground. A few might actually be volcanic rocks. None of the “rocks” in the photos are meteorites, however. None has a fusion crusts.
Curiously, quite a few of the rocks that people describe as “it wasn’t there yesterday” look like slags.