Theoretically, a small amount of Cf251 would make for a good display cube. Say a piece of foil that was 5mm to a side and just 0.01mm thick. This isotope of californium has a half-life of almost 900 years so you wouldn't need to worry about it decaying right before your eyes like most of the other synthetic elements. Well, that's the theory. Then there's the expense. The Nuclear Regulatory Commission actually publishes pricing for small batches of the metal for those who hold the appropriate licenses (Luciteria's customers need not apply). Anyway, that little foil would cost several tens of thousands of dollars to manufacture and if you wanted to flaunt your very own bullion ounce of californium you had better be a billionaire.

Then there's the safety issue. Californium's rate and type of decay make it among the deadliest substances known. That billionaire would be dead within seconds of holding his precious ounce of this element. 

Fortunately, you don't have to worry about the safety of this cube. Those golden blobs in the embedded film are harmless, we promise, and not a single atom of real californium ever touched this gorgeous cube that pays homage to ol' 98 on the Periodic Table.