Amber and turquoise are very suitable stones for buckles, and jade is the most durable. Curved nephrite is a good option, as quality jadeite in buckle size is expensive and difficult to come by. One has to select buckle stones for their strength as well as for the ease of re-polishing. Amber, even though it is soft, it is quite resilient and has the benefit of lightweight. 
A gold buckle (even a small one) is a lot of gold, and it takes creativity to minimize the weight while building a strong structure that works well, looks beautiful and delicate, and is durable.