Adamantine weapons  can be forged by only master craftsmen and for roughly 500gp + (10 x base price) and have the following properties:

• Can only be destroyed by magic or special forges that can craft such a weapon to begin with.
• +1 to Damage Rolls.
• Adamantine armor that is struck with this weapon doesn't prevent critical damage.
• Is a magic weapon for purposes of overcoming resistances.