This somewhat fanciful category includes Bronze, Copper, and Stainless Steel. Bronze has been around for a long time. It has a reputation for durability born in the Bronze Age, and a price tag similar to an automobile. They are durable, and pretty, and the top of the line for snob appeal, but all too often they are merely a plated alloy and the bottom may well be thin metal of another type which can and will deteriorate much sooner than the sides and top. Bronze and copper are listed in weight per square foot of material, usually cited as 32 oz., or 34 oz. .
Copper holds up well and, of the two, is probably the better buy.
The same problems as cited for bronze apply. Stainless Steel is included among the 'semiprecious metals' to differentiate it from the regular metal caskets. The durability of stainless steel is well known, with the caveat that there is no guarantee the bottom isn't made from some other metal. Many nice caskets are fashioned from oak, cherry, walnut, mahogany, and poplar. There are also some very nice models of pine, but you're not likely to be shown those unless you ask.