Adamantium weapons not bypassing Damage Resistance:

[Selveem]

I was wielding an adamantine weapon (an adamantite longsword) while dueling. It was the only weapon I was wielding and stoneskin still performed its normal damage reduction:

You attack Zorinar as he gets within your weapon's reach.
You score a critical hit!
Zorinar's stone skin absorbs some of your damage.
Your slash tears open Zorinar's abdomen.
Zorinar has several minor wounds.

Adamantine weapons are supposed to bypass stoneskin damage reduction altogether.

http://www.d20srd.org/srd/spells/stoneskin.htm

[Althasizor]

You wield adamantine battleaxe in your right hand.
Adamantine battleaxe feels like a part of you.
You approach Diswiz and prepare to engage him in combat!
Diswiz's skillfully lands a blow on your chest.
You cut right through Diswiz's stone skin with adamantine battleaxe!
Your slash powerfully slashes Diswiz's left leg.
Diswiz is slightly scratched.
You cut right through Diswiz's stone skin with adamantine battleaxe!
Your slash skillfully cuts Diswiz's chest.
Diswiz is slightly scratched.