i quote directly from wikipedia:
"As a powder or solid formed by
sintering (welding together small particles), alumina is opaque or
translucent. Recently, a method of sintering very small particles of alumina has been developed by researchers at the
Fraunhofer Institute for
Ceramic Technologies and Sintered Materials. This sintered alumina is very hard, nearly transparent, and has a very high melting point (2303
kelvins), yet like other sintered materials it can be produced at temperatures much lower than its melting point. In
2004, Anatoly Rosenflanz and colleagues at
3M in
Minnesota used a "flame-spray" technique to alloy alumina (aluminium oxide) with rare-earth metal oxides to produce strong glass with good optical properties. The method avoids many of the problems encountered in conventional glass forming and may be extensible to other oxides."
that is for transparent alumina....
here's another link from physicsweb and the
accompanying picture showing differnt opaques and the fact that it can be shaped, which means that a wheel is certainly not out of the question. As far as cost, there isn't any sign of it being commercially available, so cost is probably more than most people are willing to pay. I believe that those rims were simply a "proof of concept" demonstrating a possible use for the material.
another possibility is that is it composed of
ALON....but i doubt it