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"Obsidian" is the also name of the twelfth month of the dwarven calendar, covering late Winter.
 Rock short sword
Obsidian can also be made into obsidian shortswords with one log at the craftsdwarf's workshop. If you embark on an obsidian mountain or manage to reach magma before finding metallic ores, these are good at first to defend your fort from early wolf and giant eagle attacks, but weak in comparison to any metal, having difficulty cutting even mundane animal hides. Note that with the use of wood (presumably used in the handle), Elves will be be offended if you trade obsidian swords and refuse the trade.
Obsidian is the only rock that can be used to make stone-based weapons.
 Obsidian formation
Obsidian can be formed by mixing water and magma. This results in any tiles containing both water and magma to be replaced with obsidian, and produces a large amount of steam. When dumping water via a pond zone onto magma, the water has to fall free for at least one z-level (i.e. the pond zone must be two or more z-levels above the magma). Otherwise only steam will be produced.
Most non-magma-safe objects that were in the water or magma prior to the formation of obsidian are completely destroyed, which can be an effective method of removing unwanted items or enemies although long exposure times to magma are suggested to ensure destruction. All magma-safe items will be entombed in the tile, and can be regained by mining it along with some magma-unsafe items if they happen to survive the process. The survival rate of magma-unsafe items is believed to depend on the exposure time to magma being short enough to not cause the item in question to melt before obsidian formation occurs. No instances of globs of molten metal surviving obsidian formation have been noted and also the survival seems to be occurring only on hard items (items made of metal/rock) as no clothing has as of yet survived obsidian casting. Verify Obsidian farming can be used to produce large amounts of obsidian for masonry, making crafts, or construction.
- Obsidian is not currently listed as an economic stone ( > stone). You cannot switch between "use" and "not use", so you can only use obsidian for constructions if you embark in a biome with an obsidian layer. See the discussion page for a workaround. v0.31.03
[INORGANIC:OBSIDIAN] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [MELTING_POINT:13600] [BOILING_POINT:16000] [STATE_NAME_ADJ:ALL_SOLID:obsidian][DISPLAY_COLOR:0:7:1][TILE:177][LAVA] [MAX_EDGE:20000] [IGNEOUS_EXTRUSIVE] [MATERIAL_VALUE:3] [IS_STONE] MELTING_POINT:11818 <-- I think this is from the data collected by the contributors above and is more accurate than the numbers we're using, but we're using a high melting point until environmental lava effects are further along.