Nothing is known about the Lava Rocks Gun's appearance, only thing known is the fact that it was set to shoot fast small hot rock projectiles at enemies. It would be most likely obtained in the Lava Planet, but this is not confirmed. The Lava Rocks Gun had the "barrel" part which might be the same as one of the weapons Ugh Zan III held.
The Lava Rocks Gun was scrapped in very early design stages, as its firing characteristics were too similar to the XL2 Lasergun's, so, Croteam decided they didn't need both. Only known about his look is its barrel part which might actually be the one of Ugh Zan III weapons. No listing exist about the Lava Rocks Gun.
Coding for the animations being named "LavaRock01" exist Serious Sam 1 SDK.