Adamantine
Adamantine is a material that appears in SpliceHack and EvilHack. It is a material that is somewhat similar to mithril, and has different qualities dependent on the variant.
SpliceHack
In SpliceHack, adamantine is a metallic material that is 3⁄4 lighter than iron, and is more durable than mithril while being 1⁄5 heavier. Adamantine armor grants a +2 bonus to AC compared to iron armor and +1 compared to mithril, and adamantine slashing and piercing weapons gain a +3 damage bonus.
Material percentages
Items that have a base material of iron or metal have a 1⁄1000 of generating as adamantine. "Shiny" items have a 1⁄200 chance of generating as adamantine, while resonant items have a 1⁄1000 of generating as adamantine. Items of dwarvish make have a 1⁄500 chance of generating as adamantine, while firearms and horn tools have a 1⁄200 chance of generating as adamantine.
EvilHack
In EvilHack, adamantine is an extremely rare and valuable black-colored metallic alloy of adamant ore - though it is used primarily by the drow, dwarves have been known to forge and use this alloy as well. Adamantine is inherently fixed and lighter than most other metals. Adamantine armor grants a +2 bonus to AC compared to iron armor, and adamantine slashing and piercing weapons gain a +3 damage bonus. Adamantine items cost 7 times as much as their iron equivalents.
However, the method that the drow use to forge with adamantine makes it very brittle in the presence of light: forging with dark elven weapons and armor must be done with the forge square darkened, and both item types have a chance of decaying and even being destroyed while used in lit squares. In return, drow are capable of safely enchanting adamantine dark elven items to +7. Other items of made of adamantine are not subject to decay and dissolving in light, but have the same safe enchantment limit as other weapons and armor (except for elven items).
Material percentages
Non-cloth armor and weapons of dwarvish make have a 1⁄500 chance of generating as adamantine, while heavy armor and weapons of dark elven make are always made of adamantine. An amulet of reflection or shield of reflection has a 1⁄100 chance of generating as adamantine.