When trading with fletchers gravel and emeralds, your gravel is not lost.

When you trade with a flether a emerald and ten gravel, i am sure that the gravel must be converted into flint. But when i had a Stack of gravel, none of it was destroyed or turned into flint, allowing me to basically get infinite flint.

I believe that the steps to reproduce this are to
1 find a fletcher
2 make sure he has gravel+emerald= flint as a trade,
3 have a stack of gravel (it happened to me this way)
Trade w villager. For me, none of my gravel was destroyed so the flint came from nowhere..