Reagent
All spells will require reagents. They will also be used as ingredients for Alchemy and in some cases Cooking (example: Garlic). [1]
Examples of use
To create a scroll you need all the reagents for that spell plus the blank scroll. [1]
How to obtain
Some reagents will be collected via Foraging (ex. mandrake root) while others are collected by mining (ex. sulfurous ash) and still others will be collected by Field Dressing. Some can only be collected in very specific places at very specific times (ex. Black Pearls) which of course will make them quite valuable. [1]