What are the Sapphire and Gorilla Glass Weights?
How much do the respective Sapphire and Gorilla Glass displays weigh by themselves?
I'm not sure you can do a direct comparison here from a weight perspective unless you had the two parts in hand and had a very fine scale to weight them.
Let say the size of the cover glass is the same for either. So one would need to look at other properties to figure out if there was a weight difference. The easiest way is to look at the Atomic Weight of the two. While one can find Sapphire, Gorilla Glass is not listed but we can compare it to Quartz. I suspect Gorilla Glass would be heaver as the molten alkaline potassium salt would impregnate into the voids. Lets say it would add 10 to the Molecular Weight.
So looking at the numbers:
A difference of 41.88 (grams per mole) so its clear Sapphire is heaver if the objects were the exact same size in this example. Now adding the potassium salt it would only be about 32 (grams per mole) difference.
It's also possible Apple uses a thinner plate of Sapphire here given its costs and added strength. So the difference could end up being very little.