Jackman's page

And at any level that +2 damage could be the difference between a dead enemy and one that has 2 hp and gets to attack you back.

All items made out of special materials have the hardness listed for the material.
From the SRD
Adamantine
HP/inch 40 (weapons and armor normally made of steel that are made of adamantine have one-third more hit points than normal.); Hardness 20; Cost: Adamantine is so costly that weapons and armor made from it are always of masterwork quality; the masterwork cost is included in the prices given.

Notice the semi-colon. Adamantine weapons have a hardness of 20 and a a third more HP than their steel counterparts.

Jackman wrote:

Can this feat be taken by a ranger at first level?

No, because a 1st level Ranger does not meet the prerequisites. Earliest they'd be able to take it is 5th level.

The feat doesn't list a level requirement just a class ability. The argument could be made that the Ranger, even at first level, has the animal companion class ability. The feat wouldn't kick-in until you actually gain the companion at 4th level. I was just curious, because that way you could take it early(1st or 3rd level) and when you gain the animal companion at 4th level it would arrive as the tougher version, instead of waiting a whole level to toughen up.

Can this feat be taken by a ranger at first level?