Southern Arizona is mineralogically rich, and there are many metal ore mines here. Many of the smaller mines and prospects were worked during the Wild West (the 19th century) or earlier in the last century. Silver, lead, and especially copper are found here.
Metal ore is smelted (heated to high temperatures) to separate the molten metal from the lighter impurities (host rock and other minerals). These impurities are called slag, and they form a strange, glassy type of rock when they are poured off and then cooled. Piles of slag called slag heaps are found around smelters, and I often discover these when I'm exploring old mining areas.
Slag is also a derogatory slang term in some English-speaking countries (but not here in the U.S.) and it means worthless, ugly, female slut. In the past, heaps of slag (the rock) were considered to be ugly, useless trash, but now slag can be reprocessed to remove lighter minerals or crushed for other commercial uses.
Slag can also be quite pretty, and depending on the ore and host rock type, it can be colorful or coated in a lovely rainbow sheen. Beautiful carved and polished decorative objects are now made from some types of slag glass.