I found these sites to have great sections on diamond education.
I highly recommended JamesAllen.com’s diamond education as they address most problems one would be faced when buying online. Do check out their live chat too as that can save you plenty of time searching for some information on diamonds.
You might also want to take a look Diamond International’s Learning Pages. They touch on a few more topics such as how to buy the right type of jewelry and they describe what different words mean when referring to ring styles. It is definitely worth a look.