I'll have to do more reading tommorow, but from what I'm getting from this, Basalt is just extrusive Gabbro, Andesite is extrusive Diorite, and Rhyolite is extrusive Granite.
The difference between the three pairs is in their mineral composition. On one side, you have "Mafic" (a concatination of Magnesium and Ferric, which are unusually common in these stones, while sillicates (SiO2) are unusually low), which are the basalt/gabbro pair. On the other side, you have "Felsic" (High sillicate composition), which is the Granite/Rhyolite combo. Andesite/Diorite is in the middle between the two. (There's also another category of "Ultramafic" rock, which forms a Komatiite/Peridotite pairing, although neither stone are currently in DF. Kimberlite, however, is an ultramafic stone that is in the game... and that's the stone that you search for diamonds in.)
Wikipedia is talking about how this is intrinsic to the magma flows themselves, and talks about how things like Andesite (magma) is formed by mixing rhyolitic magma with the basaltic magma, which basically occur on the faultlines of tectonic plates, implying that you would not expect multiple types of magma to be available in any one location (so, you get one of those types of magmas, and that's it), although I'll have to do further reading.
If we have areas that are specifically one kind of magma, with that specific kind of igneous rock, we could specify exactly what non-hydrated and hydrated glasses they should form. If it is map-wide, it would only have to be a single variable for the entire map, making it fairly resource-light. Since these different stratifications have different chemical compositions, we could basically make this good for handling the probabilities of different ore deposits, so that iron ores are more common in mafic stones.
There is, however, a problem. Most of these hydrated glasses that are formed are, essentially, pumice-like stones. That is, they are ultra-light, and not useful for direct application as stone, although they can be ground up into a soil, or used as plaster.
I'm thinking it might be possible, however, that "hydrous" doesn't mean what I first thought it meant, based on what G-Flex said. Hydrous magma seems to just mean that there is water in the magma (before it cools), rather than that it cooled specifically because of contact with water. The hydrous glass is light and pumice-like because the water in it manages to escape the stone as water vapor as the magma solidifies... This is complicated because there doesn't seem to be a specific wikipedia page on the subject matter... I'll just do a broader google search tommorow.