Yeah my mate did that while we were on a stag do in Belgium, we went to Antwerp and picked a diamond out in the diamond district. Quite a few carats from memory and price seemed good.
It does amaze me that obviously size matters with diamonds but clarity, colour, defects and even cut matter much more to the final price.
We were shown perfectly similar diamonds to my eyes yet the price was double!
Most people cant tell a few grades in colour different and a fortune can be saved picking say one from G to I over D to F.
And some people prefer the more coloured diamonds which are the cheapest as well.
Same with inclusions. You can buy diamonds with inclusions which cant be seen to the human eye without magnification which cost 6 times less than "flawness" diamonds. If you cant see the bloody faults, apart from you knowing they are there from the certificate, does it matter?
To be fair, if I was buying a diamond engagement ring today, id go to somewhere like Antwerp and select the diamond and then get the ring made later to her design.
I did that with a necklace for the gf many years ago. She picked out a 6 carat sapphire and then she drew the design of the necklace in white gold. We collected it a few weeks later. The jewellers liked it so much they asked if they could make the design and put it on sale. She should have got commission!
Proper coloured diamonds (as opposed to "off clear/colour" diamonds) cost far more than normal diamonds.