Most items in Adom are made from following materials:
cloth, leather, stone, iron, mithril, adamantium, eternium, gold, glass, "various", crystal.
An item can only be made of one material, even if by logic it should have parts consisting of other materials as well (which is often reflected in weight; compare the weights of iron and eternium as ingots (20s v 8s) and ring mail (300s v 196s), which makes sense considering ring mail is never worn naked, usually requiring leather or cloth undergarments. Still, ring mail is treated as if it consisted of eternium or iron respectively only.
Only iron, mithril, adamnatium, eternium stuff can be smithed - these are the commonly used materials for weapons and armor. Items made of gold appear on smithing list but golden ingots do not exist.
Material has several implications:
Weight - in general, among the four smithing metals, they get lighter as they increase in quality - iron is heavier than mithril, which is heavier than adamantium, which is heavier than eternium, the lightest of all metals. Eternium weighs less than half as much as iron in its pure form, but weapons and armor will shift, presumably depending on how many of its other parts are not made of the metal that is its main component.
Stone (some weapons) and crystal (some weapons and armor) items are in general much heavier than items made of the main metals.
Elemental resistance - Higher metals [mithril, adamantium, eternium] and crystal made items are immune to fire and at least resistant to acid, while most others are not. A notable exception is stuff granting elemental resistance, which are always immune to corresponding elemental despite material used - so the ring of fire resistance (which is always made of wood) is unaffected by fire and iron made bracers of resistance are immune to fire, acid or lightning.
Artifacts are indestructable by elemental attacks regardless of material.
Iron made stuff is the only stuff that rusts in water (rivers, underwater, slipping in poools, dipping in regular uncursed water potions) or can be rusted by rust monsters. From ToEF tests iron is also somewhat fire resistant - it lasts longer than leather or wood made items but melts eventually.
Corrosion - some monsters have the ability to corrode weapons upon being hit. This mostly affects iron made weaponry, while mithril and adamantium are somewhat resistant. The only materials immune to corrosion are eternium, stone and crystal. (Attacking with something like leather armour still needs investigation).
There is such a thing as wooden weapons (simple clubs, simple quarterstaves and pick axes). These are vulnerable to corrosion, not affected by rust, do not appear as smithable stuff (so can not be fixed via forge) although they are affected by weapon scrolls normally, they also are vulnerable to acid and fire (latter gives message of weapon burning away opposed to melting away for metallic weapons). This is rather realistic behavior of wood, though one does have to wonder why pick axes belong here.