There are lots of shops selling freshwater pearls and semi-precious stones on Charoen Krung (New) Rd between the Shangri-la and the Oriental -all quite reputable in my experience. To test whether they are real -rub them on your teeth and if they feel gritty they are. Freshwater pearls are much cheaper now than a few years ago as they are farmed in China.
The Narayana Phand Bldg will be cheaper as will Chatuchak which has many semi-precious gem sellers but the quality varies at Chatuchak.