I would recommend anyone who intends to buy diamond jewellery to look at the link below and get familiar with the 3 c's. Clarity, Cut and Colour.
Cut usually determines how the stone reflects the light. The deeper the cut the more it does so and the more it costs. Not that easy to check without an eye glass.
Clarity is easy to check. There is a chart which shows what, for example, a diamond with VS1 should look like and explains it in simple terms. Basically if you ask the jeweller to borrow his loupe (eye glass) with a VS1 diamond you should just about be able to see some inclusions (bad - quite often small black marks in the stone but can be clouding also). If you see lots then it is probably a lower grade stone (S1 or I) and you should question the information they are giving you. If you cannot see any inclusions then it's either a flawless stone (which is quite rare and would be reflected by it's high price) or a fake.
Colour is again sometimes personal choice, if, for example you want a champagne or other fancy colour. However the most expensive ones are the ones that appear colourless.
I would recommend taking a copy of the chart with you as well as I have seen doctored ones in Turkey. I once questioned the clarity of a VS1 diamond as it was quite badly included and was at best an SI1 or SI2. The jeweller then showed me a chart which was very different from the official one I had at home.
Personally I would buy a loupe off ebay for about a fiver and practice with your stones at home. It's quite interesting to see the insides of the diamond and you will probably be gob-smacked at how much soap and crap your rings pick up!
ops - forgot the link
http://www.amazon.co.uk/gp/feature.html?ie=UTF8&docId=1000062973