Hi!
We think the sun is mostly made up of hydrogen and helium. There are just little bits of everything else, like carbon and oxygen. We are basing that on what we can measure from the surface and what we can figure out be applying physics to what we know of the sun's mass, temperature, age, and other things.
Astronomers do think there are some stars, called white dwarfs, which have a lot of carbon in them (although not, I think, diamonds). The sun is expected to become one of them some day, but not for billions of years. You can read more about that here:
http://imagine.gsfc.nasa..../know_l1/dwarfs.htmlcheers,
Terry