Well, you may be looking at it wrong. Why assume they are floating up?
I think its better to assume they are following Bender down deeper into the lava, and it looks like the cape is floating, but actually, its staying still.
Furthermore - Dolomite isnt hard. Its a type of carbonate, which are generally soft - (most known is calcite) Its hardness is about 4. One of the reasons it can deal with being in the magma (yah it should be magma and not lava, because its technically underground, and magma is only called lava once it has hit the surface) because magma and heat and pressure will actually create minerals like dolomite from the original rocks (changing its chemical composition)
Yah, Im a geology major
Edit: more info on dolomite http://mineral.galleries.com/minerals/carbonat/dolomite/dolomite.htm