The diamond market does not seem transparent to me. How do I know that I am getting a fair deal?

I will tell you more: even to 90% of all jewellers this diamond market is not transparent at all!
It therefore takes a good deal of informing yourself about where to pay attention to, as I stated in the previous answers.
Always bear in mind that when you think you can do the diamond deal of your life, you always get what you pay for!
Fair prices exist like on this site, bargains do not, despite many others may try to convince you otherwise.

For further information see Diamonds Glossary.