Blackrock
I made up an adamantite substitute for my Britannia 3E setting, based on blackrock:
Blackrock and blacksteel
Blackrock is a strange ore with properties of magic resistance, found in seams deep underneath the Serpent's Spine mountain range. When refined, it yields blacksteel, a shiny, pitch-black metal that is dense, heavy and ultrahard. Blacksteel is nearly impossible to work, but the master smiths of Minoc have recently discovered the means to craft it into weapons and armour. These items benefit from blacksteel's extreme hardness, as well as its ability to disrupt magic.
Any suit of armour crafted from blacksteel has a natural enhancement bonus of +1, +2 or +3, depending on whether it is light, medium or heavy armour respectively. The armour also grants spell resistance to its wearer when worn. A suit of light armour grants SR 15, medium armour grants SR 17, and heavy armour grants SR 19. A shield crafted from blacksteel is treated as light armour for the purpose of determining its natural enhancement bonus and SR.
Any weapon forged from blacksteel has a natural enhancement bonus of +1 or +2. Weapons with a base damage of 1d4 or 1d6 gain a natural +1 enhancement bonus, while those with a base damage of 1d8, 1d10, 1d12 or more gain a natural +2 enhancement bonus. When a blacksteel weapon deals damage, the target is affected as though by a targeted dispel magic, cast at 5th caster level. This is an extraordinary ability.
Blacksteel has hardness 20 and 35 hit points per inch of thickness. Natural enhancement bonuses do not stack with any other enhancement bonuses; thus, a blacksteel (+2) longsword enchanted with a +5 enhancement bonus effectively has a +5 enhancement bonus. In an area where magic does not function, it still retains its natural +2 enhancement bonus. Blacksteel items are treated as masterwork items with regard to creation times. The masterwork quality does not affect the enchancement bonus of weapons or the armour check penalty of armour, nor does it add to the market price.
It should also be possible for other items besides weapons and armour to be fashioned that take advantage of blackrock's unique properties. If any such items have been created, their existence is not common knowledge.
Market Price: +32,000 gp (light armour or shield), +55,000 gp (medium armour), +80,000 gp (heavy armour), +33,000 gp (weapon damage 1d4 or 1d6), or +39,000 gp (weapon damage 1d8, 1d10, 1d12 or more).