Flint

Flint is a item from Gravel. While it has various uses, its main use is to create Enchanted Flint, which is used to upgrade a Gravel Minion from Tier V onward.

Mining
Flint is a possible drop from mining Gravel in the Spider's Den.

Minion
It is collected when using a Gravel Minion, and can be collected more reliably using a Flint Shovel (both manually or as a minion upgrade).

Purchase
Another way of obtaining is buying them from Pat. Their shop is at the Graveyard they sell cheap flint and gravel.