Hi, there are a few shops in Samui which can jewellery make by order , I personally had a semi-precious stones experience with the shop Classic Gems in Chaweng opposite Centara Grand resort, and the jewellery they made are beautiful (mind you I did it with white gold, not silver, but I am sure they can do silver too). Also another shop ( sorry forgot the name) - is just a few meters down the road from this one, there is a lovely Chinese guy who can also make stuff (he made for me a copy of my white gold earring which I managed to lose).
You can also check a few silver shops in Chaweng, I particularly like 3 of them - one called Try Out - small white colour shop on the corner opposite Buri Rasa resort, then a very small shop called Jantima tucked between massage shop and something else near TOPS supermarket in Chaweng, and also one shop just in a beggining of Chaweng, kind of opposite Muang Samui resort.. Hope it helps somehow.
Also check out my blog about Samui www.samuiconnection.blogspot.com
Elina.