The "lava" being transparent makes it look like boiling tea rather than lava - it looks almost like you should be able to swim in it for a little while before it kills you, rather than the insta-death that it probably is.
Molten rock and molten metal are opaque. Plus, they're that colour because they're glowing, so even if they were translucent, the glow would still make them effectively opaque because it would drown out anything darker that's seen through them.
Edit: This it off-topic, but I love the crane as a level design element, since it gives the player both a quick pathway that requires control finesse, and a slower way that doesn't.