It is a large, gray sphere, appearing to be made out of steel or another heavy metal. There is a hole at the top of the device, and four thick metal claws used to hold the Precursor Stone in place.
There were two piercer bombs, the first at the weapons factory (without the Precursor Stone), almost ready to be shipped off in an eco tanker. It was destroyed after fighting Krew—the resulting explosion killing Krew as well. The second one was hidden at the construction site. This one did contain the Stone. Somehow Daxter managed to deactivate it from the inside-out.
The piercer bomb was powerful enough to crack open the Precursor Stone, according to Krew. In result it would release the untold energy inside (actually destroying the entire planet) and blow up the Metal Head nest along with Metal Kor and most likely the entire main forces of the Metal Heads located there. They had planned to move it there with a shipment of dark eco, but Jak interfered both times.