Bi metal is sterling silver with a mechanically fused layer of gold which is about 3x thicker than plating. With friction the gold will eventually start to wear away and so this material is best not used for rings. How quickly it wears depends on the way it is treated and also (and this applies to gold plated jewellery also) to the checmical makeup of the wearer - ie some people will cause gold plating to erode more quickly than others.
Earrings by Susan May in bi-metal