What determines the transparency of a substance?

Ultimately, it has to do with whether or not there are electronic transitions (changes in energy state) that have an energy that corresponds to a frequency in the visible range.

There can be transitions due to intra-atomic/intra-molecular state changes, or due to state changes among electrons that are "floating around" in the solid. I think most colors in crystals have to do with intra-molecular absorption: this is why rubies are red, for example.

This can't be true for graphite and diamond, because they are both built up of carbon atoms. To figure this out, you have to calculate the so-called "band structure" of the electronic states of the two. These calculations will be very different between these two structures: for one obvious point, the graphite crystal structure is 2-dimensional (with stacks), whereas the diamond structure is definitely 3-dimensional.

You can study this more by searching under the term "band structure", but I don't think there is a succinct answer to your question: The calculations have to be done.

make it simple.do not think to to far into it.....lets just say light must be able to travel through an object for it to be considered transparent. That is her thought. My thought is the complete removal of color..

Please reply with your thoughts. Thanks!

John B ... Is that you? be afraid, be very afraid!