Flint

From Witcher Wiki

Flint is used to start fires when Geralt cannot use the Igni sign.

I'd suggest keeping up to ten of these in inventory at all times, but no more. Ten will stack and take only one inventory slot. I also consider it one of the most disposable items in the game. If I need to free some space in inventory quickly to pick up a more valuable or quest-related item, they are the first item I sacrifice. When I have the space however, I don't waste them and use Igni whenever possible as sometimes the sign is just not an option, i.e. in town during daylight hours, whenever the area is considered "safe".

[edit] Price

Buy	5
Sell	1

[edit] Source

• can be bought wherever jewelery and gifts can be bought