Jewellery forms are mostly made from silicone.
We use these forms (moulds) to cast our jewellery from urethane resin, epoxy resin, Art Clay etc. These materials do not stick to the silicone at all. So as any other materials.
Nevertheless, the silicone moulds also play an important role in the casting of classic metal jewellery. Because, for the metal casting, a sample from wax must be made first, which is also casted into a silicone mould.
This process requires a special silicone that can withstand the temperature of the melted wax. From other aspects, the jewellery moulds should be durable and have a very good and accurate reproductive ability.
That means that those must be able to return the original master copy with an accuracy of micron mm. In addition, the jewelleries are generally chiselled with fine patterns, so the forms should be designed the way that the casted piece can be easily removed from.