They said that transparent aluminum was not possible.
Well, ALON is in fact real.
Aluminium oxynitride or ALON is a ceramic composed of aluminium, oxygen and nitrogen. It is marketed under the name ALON by Surmet Corporation.[3] ALON is optically transparent (≥80%) in the near-ultraviolet, visible and midwave-infrared regions of the electromagnetic spectrum. It is four times harder than fused silica glass, 85% as hard as sapphire, and nearly 15% harder than magnesium aluminate spinel. Since it has a cubic spinel structure, it can be fabricated to transparent windows, plates, domes, rods, tubes and other forms using conventional ceramic powder processing techniques. ALON is the hardest polycrystalline transparent ceramic available commercially.[2] Its combination of optical and mechanical properties makes this material a leading candidate for lightweight high-performance transparent armor applications such as bulletproof and blast-resistant windows and for many military infrared optoelectronics.[4] ALON-based armor has been shown to stop multiple armor-piercing projectiles of up to .50 BMG cal.[5] It is commercially available in sizes as large as 18 by 35 inches (460 mm × 890 mm) monolithic windows.[6]
https://en.wikipedia.org/wiki/Aluminium_oxynitride
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