Art Clay 650, with its low firing temperature, can be combined with cubic zirconium, a substitute for diamond. The chemical symbol for cubic zirconium is ZrO2.

Cubic Zirconium is man-made: typically it's a combination of Zirconium Oxide and Yttrium Oxide, both of which are normally opaque. However, when melted together at over about 2750°C and then cooled under controlled conditions, they form cubic zirconia. Cubic zirconia is colourless, so colours are made by adding small quantities of other materials.

It's not easy to tell the difference between cubic zirconia and real diamonds, unless you know what to look for or can compare the weight. For example, cubic zirconia is 65% heavier.