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From the site...
"A ceramic research lab in Dresden, Germany, has developed transparent Alumina by subjecting fine-grained (I'm guessing extremely fine-grained) aluminum to a whopping 1200 degrees Celsius ...the result of which is amazingly light but three times tougher than hardened steel of the same thickness, and it's see-through."
Actually, for the technically inclined, the real Physics/Chemistry is Here
Excerpt...
"Rosenflanz and colleagues started by mixing around 80 mole % of powdered alumina with various rare-earth oxide powders -- including lanthanum, gadolinium and yttrium oxides. Next, they fed the powders into a high-temperature hydrogen-oxygen flame to produce molten particles that were then quenched in water. The resulting glass beads, which were less than 140 microns across, were then heat-treated -- or sintered -- at around 1000°C. This produced bulk glass samples in which nanocrystalline alumina-rich phases were dispersed throughout a glassy matrix. The new method avoids the need to apply pressures of 1 gigapascal or more, as is required in existing techniques."