Shopping for jewelry in a third-world country such as Cambodia offers numerous advantages -- such as cheaper labor and closer proximity to certain gemstone sources -- that make it substantially less expensive than shopping for similar items in the West. However, Cambodia is also a country that has a lot of order and very little law, so a shopper needs to be very wary, indeed, when it comes to issues like product quality, scams, and fraudulent goods.
Work the travel forums. You are not the first person to wonder how to buy good jewelry in a "wild west" location like Cambodia, and the specific advice of recent visitors on which merchants to avoid can prove golden.
Your greatest risk in shopping for diamond jewelry in Cambodia is buying fake gemstones. One tip: Closely examine the jewelry setting -- it is highly unlikely that a real diamond would be set into a high-quality gold ring, for example.
Go to Phnom Penh. As Cambodia's capitol and only major city, it is really the only place to be if you are shopping for diamond jewelry in that country.
Go shopping. If you have had no luck in getting recommendations before visiting Cambodia, there are two main locations to find jewelry merchants: in and around the Central Market, and on the Mekong Riverfront. As any recommendations are likely to be one or the other or both, shop at other merchants while you are there. This will help you compare both what is out there and prices, but remember that the only logical explanation for a steep cut in prices is that you are dealing with a fake.
Cambodia's market culture is always about haggling. The best approach for an item like diamond jewelry is to decide what you are willing to pay and insist on getting it. If you can get less, even better, but don't fixate on achieving the best possible bargain. If you are happy with the price paid and paying less than you would at home, it should be enough.
Never accept recommendations from tuk-tuk/taxi/motobike drivers, your guest house's staff, or touts on the streets. This will always lead you to a merchant looking to sell you fakes. You always want to be in control as the person who initiates any business proposal.