I'd like to correct one small detail. As a fan of caves and spelunking, I am well aware of whats realistic in that reguard. And underground rivers and streams are very realistic. Also extremely dangerous. And yes, like surface rivers and streams, they do have sand in them, as well as clay and other things. I live not 50 miles from two caves that have, or did have, underground rivers in them. In fact, all limestone caves were created from flowing water, as well as many other caves. And I know of a cave in alaska, that was cut thru basalt, by meltwater from a galcier. The water was trapped by the glacier, creating a small lake with no outlet. The water found and flowed thru a fissure in the basalt. And sand carried by the water, sanded away the sides of the fissure, until the cave is now big enough to drive a car thru in places. A mountain cave in Austria was also cut this way, and now has a verticle shaft over 5,000 feet straight up and down, and so wide that people have PARACHUTED down it!
Also I dont know much about glass or glass making, but I do know a bit about pottery. Pottery was one of man's first inventions, followed not long after by glazing. And you can ask any potter that has ever worked with any glazes, that you cannot tell the color of a glaze, by what it looks like before it is fired. Almost all glazes are made of silicates for glass, and metal oxides for color, and they almost all change color when fired. And you get some very unexpected colors from the oxides used. People have been using and experimenting with these things for thousands of years now, so some colors are very common knowlege. Yet we are still experimenting and coming up with new glazes. Dwarves, with their long lives, rich histories, and hands-on experience with all kinds of ores, sands, clays, and rocks, would certainly know many basic colors of glass and glazing. Certainly more than could be easily shown in DF.
A magma river tho, that is very much a stretch, to say the least, unless they are living in a volcano, which would not be the best choice of homesteads. They could certainly find some if they dug deep enough, but they would likely be roasted long before hitting any. And getting it to the surface in any usable form or amount would be near impossible. But its a GAME, not real life. And I like my steam defenses, so I'm glad for the stretch!