Transparent aluminum is a form of aluminum that is see-through.
Most commonly someone speaking about transparent aluminum is referring to AION (aluminum oxynitride), a ceramic alloy. However, aluminum can exist in an elemental, metallic form made transparent by bombarding with a soft x-ray laser.
In Star Trek IV, the Enterprise’s engineer, Scotty, barters with a materials scientist using his future-based knowledge of transparent aluminum, one-inch of material as strong as six inches of the their polymer-based material.
In reality, AION does represent a great increase in strength: applications in bullet resistant “glass” create transparent armor that can stop an armor-piercing .50 calibre round travelling 2700 feet-per-second, at less than half the traditional thickness of bullet-resistant glass that would fail to stop the same round. In this application, it is used as a layer in an AION glass and polymer sandwich.
AION is difficult to produce: aluminum oxynitride powder must be pressurized to 15,000 pounds per inch in rubber molds submerged in hydraulic fluid. Then the molded and still opaque material is heated to 2000’C and kept at that temperature for two days. The AION emerges transparent after cooling and is ready for a final grind and polish for extra clarity and strength.
It has been argued transparent metallic elemental aluminum was really what Scotty said was “the ticket, laddie.” Even recently many claimed this material would never be a reality. Oxford University scientists claim to have discovered that aluminum subjected to brief pulses of the FLASH laser, focused down to an area smaller than a cross section of a human hair, made it transparent. However, the effect only lasted forty femtoseconds and each pulse of the laser required enough power to supply a whole city.
See a video about transparent aluminum:
See Scotty introduce transparent aluminum on Star Trek: