Someone should probably post the ones these other guys are talking about. Failing that, I heard there's a good exotic-material system in Heroes of High Favor: Dwarves (written by Wulf Ratbane, of Wulf's Story Hour)
I use my own version, which is more of a generic "Material" ruleset to replace the Masterwork and material rules from the PHB/DMG.
The general idea was that every material has a Hardness, a DC modifier, two Cost multipliers (one armor, one weapon), and two lists of benefits (one armor, one weapons). Obviously, certain materials can't be made into weapons (leather) and certain can only make specific types of armor.
The base rules give you a normal steel or good iron weapon. Fine Steel (i.e., masterwork) is 10 Hardness, +3 DC, x10A/x20W cost, and give benefits of (+1 attack for weapons)/(+1 Armor Check for armor). So, take the Craft DC for a normal weapon, add 3, and you can make a "masterwork" item that adds an extra bonus for a higher cost (not a flat rate).
Dragonscale might be 30 Hardness, +16 DC, x150A cost, and give benefits of (+2 AC, corresponding elemental resistance only costs +2, 3/4 weight, +2 Armor Check, +2 Max DEX) and can only be used for Scale Mail or Plate-type armors. I'm just making up numbers here, the document is at home. Hey, if you can find an armorsmith who's comfortable with a check DC in the 30s for an item with very rare components, more power to you.
(There's also Dragonbone and Dragontooth for weapons, and Dragonskin for leather armors)
It's like adding a Template to the weapon. It makes it easy to make a rare, complex item; everything from a simple bronze longsword to a Gartine Scimitar to "Ooh, a +3 Orichalcum Spell-Storing Jambiya of Sharpness!"
In general, the weapon cost multiplier is twice that for the corresponding armor; Mithril might be x30 for armor and x60 for weapons. Reason is, a mithril weapon is almost all mithril, while mithril armor is usually built over a more mundane base, like a padded or leather suit. Same goes for Dragonscale; it's not solely made from dragonscale, it uses a chainmail base. Exact values vary.
Most of the specifics won't help you because they're not quite balanced with the core rules; I use that different materials give a Material bonus to various things, which lets them stack with magical enhancement. In general, this makes armor and weapons stronger, which penalizes non-weapon classes, but it's to counteract changes I made to spellcasters IMC that made them stronger.
One downside is, it becomes cheaper to find rare materials for small weapons than to heavily enchant a large one. For the price of a +4 longsword, you could buy a dagger out of really rare materials and enchant it to +3 to get a higher overall damage. Wait, did I say downside? That's one of the things I like about it. If the players can find those materials, it makes for nice plot hooks.
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