Polymer clay is a synthetic modelling material that has been used by artists for the several decades. For a fascinating account of the development of polymer as an art material, you can’t go past Elise Winter’s PolymerArchive site.
When it is raw, polymer clay is soft and pliable and you can stamp it, texture it, mix it with other pigments and materials and blend it to create new colours. It becomes hard when cured which can be done in an ordinary oven at relatively low temperatures. After curing it can be carved, scratched, sanded, painted, polished and further altered. It comes in a range of colours although the colours I use are generally hand mixed from the standard colours.
There are many brands of polymer and different types are good for different styles and conditions. I currently use Kato and Cernit clay.
Jewellery made from polymer is durable, fade-resistant and water-proof. Use your common sense and give your lovingly hand-made jewellery the respect it deserves. Polymer can be cleaned using a soft cloth and warm soapy water if required. Rubbing with a rough towel will restore the shine on a previously polished piece if it has dulled. Do not store cured polymer jewellery with uncured clay. Do not wear if you are using milk based mosquito repellent. Ask me how I know!