In article , Brion K. Lienhart
wrote:

Nope. F:2.8 is solely based on the size of the lens. Obsidian is glass,
you can grind it into a lens shape, but if you use it as an element in a
lens, you're going to get some reallllllly slow shutter speeds. Of
course that's an extreme case, the difference between plain old glass
glass, and exotic flouro-silicates is only a few fractions of a percent.

An f/2.8 lens is going to allow a specific amount of light to go
through regardless of what kind of glass it's made of.