Hi, wife of chrisSZ here. otherwise known as ballast, (who let a women on eh?).
Been selling diamonds for about 9 years now so hope i can help. work in swag bluewater but on maternity at mo (so not on commission and will tell you the truth!).
If you're looking for anything bigger than quarter carat, quality is definately important. Swag don't sell much lower than H SI even with no cert. certs are always worth looking at. And always ask to see the stone under a 10x magnification. Anything above SI you're unlikely to be able to see inclusions.
Cut is vague on certs as proportions are a complicated area. Ask too see a perfect cut 'hearts on fire' (a brand) as a comparison if you're looking at round stones. Once you've seen and understood stones with a cert it's easier to know what you're looking at with non certs. personally for something really special i'd go for G VS but it depends what you're after. Graeme's a good guy. Deputy manager. Give him a shout. SWAG 01322 427829.