Well, something like silicon carbide might be. Its nowadays produced in gemstone quality and very much like diamond (even higher refractive index, almost as hard), and its fire resistant - at least to temperatures seen in house fires. When cut properly no lay person is likely to tell the difference, apart from the fact that these things can be produced more or less to arbitrary size.
In any case, diamond will lose its value over the coming decades. Synthetic technology keeps on advancing, and i'm quite sure will see a larger lab grown diamond of gem quality than nature ever managed to surface.
In that sense, gold is more 'forever' than diamons are, since its abundance is known to be low, and sythesis will remain very expensive.
