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.
It, like Amber, is a very lightweight material and can easily be carved with a knife. Its associated with the Moon, and the Romans associated it with deflecting the Evil Eye.
The implications are obvious here: such a sharp material can be used to sever ties between people or things, both on the physical and spiritual levels.
Be warned, however. Overuse can cause folks to become antisocial and withdrawn. Its definitely a solitary worker, in that respect.
In its clear, crystalline, unpolished variety, its a memory stone. It magnifies and projects energy. When used appropriately, it can amp up the energies used in protective work.