About rubber
Rubber parts are most commonly found as packing material of various types. Normally, this is for sealing edges along the periphery of the luminaire against a glass disc, for instance.
It is also found in insert seals for cables. More recently, plastic qualities (elastomers) which are reminiscent of rubber have taken over a number of these applications. They are cheaper to produce and can be made with greater precision. They also provide a more aesthetically pleasing surface than rubber.