Before I talk about the problem, however, here's a little background on spinel. It's a actually made up of magnesium aluminate (MgAl2O4), which means it's super tough. It has a Mohs Scale, or toughness, rating of 8.0.
To compare, diamond is 10.0, sapphire glass is 9.0 and regular glass is around 5.5. That means spinel will hold up to shock, strain and erosion better than glass. However, it can be made clearer than sapphire and it's much less expensive than diamond.
The potential uses for spinel are endless, from super tough smartphone screens to thinner bulletproof windows. Spinel doesn't block infrared light, which makes it great for lenses and windows on military hardware.
The weight savings for vehicles would be excellent, especially with the stricter mileage standards on the way. Plus, you wouldn't have to replace your windshield nearly as often, which would be a big money-saver for those of us who live in the desert.
So, why isn't spinel in everything? Until recently, it couldn't be made in pieces larger than 8 inches in diameter, which really isn't that big.
However, the U.S. Naval Research Laboratory figured out a way to make it in 30-inch sheets that's much less expensive. The key is sintering the base powder using a large press, which also lets them shape it.
The only thing left to do is figure out a good way to polish the spinel for clarity. That's the most expensive and time-consuming part left. If they can work that out, then you might be saying goodbye to glass and hello to the future.