Quote Originally Posted by Xuc Xac View Post
Obsidian is just glass and it behaves the way you'd expect glass to behave. "Other igneous rock" includes a broad range of materials. It's like asking "how strong is stuff that comes from the ground?" Some igneous rocks will crumble to sand under the pressure of your hand. Some are very hard and tough like granite.

In general, the most durable minerals are the ones that have the lowest melting point. The ones with a really high melting point are unstable at the temperatures on the Earth's surface because it's like being "superchilled" and they crumble more easily. Rocks are made of minerals, so tougher rocks are made mostly of those low-melting point minerals like quartz and feldspar. Igneous rocks are the ones that formed from a liquid state. The slower the rock cools and solidifies, the larger the grains will be. Cool it very very slowly (over centuries) and you can get something really grainy like granite with noticeable flecks of different colors from the different mineral crystals. Cool it more quickly, and the final composition has a finer-grain like basalt. Cool it extremely quickly (using lava as a replacement for concrete) and you don't have any grains at all: that's obsidian.

Also, unlike what you see in the movies, you can't stand next to lava inside a volcano. It doesn't just burn you when you touch it. It radiates heat that will burn you if you just get close to it. If you stand next to a river of molten lava without protection, you'll blister and burn just like standing in a fire. Those dwarves couldn't just put railings and yellow caution tape around the lava to keep people from stepping in it. If there's lava still flowing, then the place is uninhabitable.
Oh.
How would steel and other metals fare?
If it helps, the structure is made by houses carved out of the surrounding basalt volcano walls, connected to each other by walkways. The structure is carved in the area below the edges of the hole at the top if the volcano.
This volcano resides in a sinkhole, acting like an iceberg: the top of the volcano is above land, rest is below ground. The duegar have carved a path into the volcano from a underground cavern.