Patents related to aluminium oxynitride include:
Aluminium oxynitride having improved optical characteristics and method of manufacture TM Hartnett, RL Gentilman U.S. Patent 4,481,300, 1984
Process for producing polycrystalline cubic aluminium oxynitride JW McCauley U.S. Patent 4,241,000, 1980
Transparent aluminium oxynitride and method of manufacture RL Gentilman, EA Maguire U.S. Patent 4,520,116, 1985; U.S. Patent 4,720,362, 1988
Transparent aluminium oxynitride-based ceramic article JP Mathers U.S. Patent 5,231,062, 1993
https://en.wikipedia.org/wiki/Aluminium_oxynitride
...it is the production process that is expensive: https://www.azom.com/article.aspx?ArticleID=8095
Let's assume that we wouldn't build a small aquarium out of Alon, but instead let's deal with a standard large aquarium size of 180 gallons. A typical 180 has dimensions of 72 inches long, 24 inches high and 24 inches wide – this means we need three panes that are 5,184 square inches combined and two end panes that are 1,152 square inches total. Since Alon has an average price of $10 to $15 per square inch, the total cost of the Aluminum Aquarium would be $63,360 to $95,040 for the aluminum ‘glass’ alone!
Once an asteroidal source of the raw materials is found (I’d guess at least one of Mars’ moons is rich in them) we’ll all have transparent aluminum phones, windows, cars, and toilets. :^)