Au (Gold) Small portion
A yellow material that can be mined with the pickaxe.
According to the material's image name in the game folder from Alpha 35, OREAU-0.TGA, it would seem to be gold.
Now, one can only speculate about what uses it will have in the future as it is unusable in Lords of Uberdark Alpha 37.
Could be possibly used to create jewellry and/or decorations.
It would seem that the extra pickaxe and axe available to equip in Alpha 37 are made of this material.