Has anyone considered the potential to use something like the tree and plant code for crystals? The idea so overwhelmed me I felt like writing a separate post about it, but it's almost certainly been brought up before. What I want to point out is the similarity with this topic.
Crystal formations of minerals would be distinct from their rock-wall versions in many ways, though perhaps they'd still be mineable or gatherable for decorative/crafting/lighting (/food?/pet?!) purposes. Growing and harvesting crystals can potentially be very dwarfy, especially if poisonous chemicals and extreme temperatures are involved. Truly giant crystals might not grow naturally during the time scale of the game, unless some kind of magic or dwarvery interfered. Then again, who knows? Maybe crystals grow like chthonian trees, competing for cavernspace with fungal colonies, angry beasts and deep civilizations.
These crystals might be represented by a variety of different passable and impassable tiles, some of them looking like giant spikes sticking through space over several squares and Z levels in random directions, others like boxes or spirals. Multi-tile formations could be generated in a way reminiscent of the tree ideas listed by others here, or with even more varieties. Outcroppings could come in many shapes and sizes, starting with the
classic giant prisms slicing through geode-like air-/water-/magma-/toxic gas-inclusions under the ground (maybe a cavern lair substitute for some number of embark squares? Maybe poking into a cavern/the magma sea?). Wikipedia has a beautiful
gallery of crystal habits that might accidentally make you want to be a geologist.
I suspect that if the tree code, whatever it ends up being, can be modified and applied to crystals, it can also be used as an engine for even more exotic modding in the future. Toady did mention extraplanar worlds in some DF talks. Having a natural-growth engine ready for things like fire pillars or bone mountains or... I don't know, goldfish spleen thrones and other dynamically generated planar features... would probably be pretty nifty.