Protection is an easy subject to start with as its all a matter of how you handle your business. If things go wrong or you didn't do enough work, or you didn't handle the work properly, the effects are pretty obvious. If you handled the workings appropriately and all went well, life should improve for you. The proof is in the pudding...or, in this case, the proof is in the response.
Having said as much, now that we've covered herbs, prayers, spirits, etc. and their uses with regard to defensive magic, we can now get into the subject of protection through the use of stones and metals that are amiable to that end.
Like plants, minerals have their uses too. Each has their own properties and associations, both elemental and planetary. As such, they too are building blocks that can be used to spin your spells.