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 and they sell cheap flint and gravel. This is often cheaper than buying from the Bazaar and profit could be made from buying Flint from Pat and reselling to the Bazaar.