Choose carefully and don't allow the sales man to pressure you into buying anything at the time they are very good at that. Take your time to choose even going back a few days later.
The quality of stones in SL is excellent. There is one jeweller in Negombo who is no doubt the best in Sri Lanka for his quality, design & style. I have bought a few very expensive items from him and would not think of looking anywhere else, but even then I haggle with the price and get the deal I want.
Taken the items back to the UK and they are valued much more than the price I had paid.
Just be careful with the sales man and get just what YOU want.