Make ROCK harder to get, greatly increase ACID resistance

This helps prevent ROCK from appearing in normal situations, which can easily get to pressure > 2 just from the movement of the falling LAVA.

src/simulation/elements/FIRE.cpp

@@ -131,7 +131,7 @@ int Element_FIRE_update(UPDATE_FUNC_ARGS)
 				}
 			}
 		}
-		else if (parts[i].ctype == PT_STNE && sim->pv[y / CELL][x / CELL] >= 2.0f) // Form ROCK with pressure
+		else if (parts[i].ctype == PT_STNE && sim->pv[y / CELL][x / CELL] >= 30.0f) // Form ROCK with pressure
 		{
 			parts[i].tmp2 = RNG::Ref().between(0, 10); // Provide tmp2 for color noise
 			parts[i].ctype = PT_ROCK;

src/simulation/elements/ROCK.cpp

@@ -25,7 +25,7 @@ void Element::Element_ROCK()
 	Flammable = 0;
 	Explosive = 0;
 	Meltable = 5;
-	Hardness = 70;
+	Hardness = 1;
 
 	Weight = 100;