I am going to guess this is a suggestion for a mutation that grants adamantine's ability to ignore hardness 20 or less? It is already in. Check the Special entry of the Smash mutation in the Construct List, it has a special interaction with Material Damage Reduction. Also allows you to build tunnels into mountains using no better tool than your clenched fists.I'll need to HB some adamantine tips for a spider-droid mutator4.
Choose one natural attack, that attack ignores 15 points of object hardness, if it already has this mutation ignore 5 more points of hardness and deal +1 damage.
You may take this mutation once every 6 levels.
Having a burrow mutation pre-req might do nicely, and under the construct mutation list.