Magma-safe materials

Magma-safe materials are those which will not melt, burn, evaporate, or otherwise take damage when in close contact with magma. Most frequently, this comes into play when using floodgates operated by mechanisms, or when operating a magma pump. In the game, magma temperature is exactly 12000 °U.

Constructions of any material can never melt or burn. Non-constructed ice walls/floors/ramps/stairs will melt if exposed to sufficient heat.
Not submerged (that is, not opened to let magma flow over/past/around them), doors, floodgates, and raised bridges (provided that there is no magma on the space the bridge would occupy when lowered) of non-magma-safe materials are safe. So long as they are just in contact with magma, only acting as a passive "wall", they are fine. If opened, they will melt.
Raised drawbridges have a notable exception, in that allowing magma to flow over the center of the area that the bridge would normally occupy when lowered will cause the bridge's components to heat up and potentially melt.
A pump made with magma-safe material for pipes, screws and blocks is fully magma-safe. Pumps containing any item that is not magma safe will be destroyed after prolonged operation.

Full List

Stone

Alunite, Anhydrite, Basalt, Bauxite, Calcite, Chert, Chromite, Dolomite, Gabbro, Ilmenite, Kaolinite, Mica, Obsidian, Olivine, Orthoclase, Periclase, Petrified wood, Pitchblende, Quartzite, Rutile, Sandstone, Talc

Metal

Adamantine, Iron, Nickel, Pig iron, Platinum, Steel

Ore

Cassiterite, Galena, Hematite, Magnetite, Native platinum, Sphalerite

Wood

Nether-cap

Leather

Fire imp, Dragon

Bone

Fire imp, Dragon

Glass

Green glass, Clear glass, Crystal glass