We went to SJ jewellery few weeks ago and bought something. They have a big range of jewellery from something simple to something ornate, Western or Asian designs. There is something for everyone there but I suggest you turn up as soon as possible in case you need modification.
They have their factory in house.
They can always copy a design, it is only a matter of finishing it in time.
Apart from the ones on display you can ask them for other pieces they kept in their safe.
Don't forget about their complimentary pickup service.
Happy shopping!

BnS has good info.

Plan to have them pick you up your first morning there. I always call about 9:30, and they send a car for me and I manange to get there (from the Royal Orchid) soon after they open at 10:00. When you call, tell them you would like to see Lily, as you've been referred by Kathie from Seattle. (reiterate this uppon arrival; if Lily isn't there, appear somewhat disappointed, but they will assign you to someone else to help you and it will be fine).

As mentioned above, they have a wide variety of designs and they do lots of custom work. If you have a photo, they will copy it. If you are having a piece copied, do look at stones and choose the stones you want. If you've never done this before, it's fun and you'll learn a lot about the stones. Sapphires are the biggest bargain (in many colors) and rubies are next. Most jewelry is made in 18k gold.

Whether they can do this in three days depends a lot on what you are having done and how heavy their workload is at the time. Will you be back in Bangkok at all? I have often had them deliver custom work to me at my hotel.

Many thanks to both of you for your comments and advice. I think I will sort out a design before I go and try emailing them with a photograph. It should be relatively simple to produce as what I am looking for is either single stone earrings or a solitaire ring in sapphire. I was going choose diamond but after watching the new De Caprio film "Blood Diamonds" that has put me right off of buying diamonds.
finally, Kathie, Thanks for the intro, I will certainly mention your name and ask for Lily when I call.

Crellston, I wouldn't send them an email design in advance - too much opportunity for misunderstandings. If you are looking for something simple, they will have lots already made up and they can create a simple solitare ring or earrings quickly and easily.