The word 'protection' obviously encompasses a wide variety of meanings, which is possibly why many articles and books get mixed up over this and use it to mean one kind of protection whereas actually it means another.
A classic example of this is Black Tourmaline ('Schorl'), I have covered this in a separate blog piece, but in summary Tourmaline does not protect the human energy field. On the contrary, it attracts negativity to itself, so if you wear it you are also attracting negativity (arguments, bad feelings etc.) to yourself. It is a very dense crystal, as is clear by its colour (a very dense black) and how opaque it is (no light can travel through it). Like all black crystals, and like the colour black itself it therefore absorbs all the wavelengths of light. In the same way it can absorb/protect you from the unpleasant energies of your mobile phone and home wireless. Just place a small piece with either and kindly ask it if it will take in the negative electromagnetic smog so you don't have to. In this sense, yes it is protective!
Another black crystal that is also mistakenly called protective is Obsidian. Again, the properties of this crystal are misunderstood. This crystal is deeply powerful and if working with it, it can show us our fears, and our own 'shadow'. It will not protect us by blocking out negativity. Instead, it will throw our own fears and issues at us, so that we can work through them and clear them. It can bring deep hidden issues to the light. If we have totally worked on ourselves and have no more fears or 'shadow' left, then yes, it is deeply protective, similar to being a 'black belt' in karate, we are then master over ourselves, and nothing can then touch us. However, for the rest of us mortals, who are still working on issues, it's a crystal to be treated with great respect!
So which crystals are 'protective'?
Amethyst is highly protective, because of its violet colour, which vibrates the fastest of the visible colour spectrum. It is a deeply spiritual stone. To me, its like having Angels nearby looking over you. As well as being protective it can also clear the energy field of negativity ('the violet flame'). It is also an excellent stone to have under your pillow to give you a peaceful sleep, and can help with nightmares.
Hawks Eye (also called Blue Tigers Eye), also quartz, is a very close relation to Tigers Eye, and can sometimes be found together in the same stone. This crystal, rather than a golden sheen has a deep blue sheen. It is purported traditionally to be of help in easing aches and tensions, and rather than the mundane, it is protective on the subtle levels, from the things we can't see. For this reason it is a good stone to have to protect oneself psychically if we feel 'too open' or under attack on the subtle levels.
Sometimes Lapis may be too much for someone, as it is so profound, but there is another stone (which is part of the constituent mineral make up of Lapis), which can also be used on its own, and this is Sodalite. Sodalite, is not such an intense deep blue as Lapis, nor does it have the gold flecks of Iron Pyrite that Lapis contains. However, because of its deep blue colour it is still protective, and because of its cubic structure, it is an excellent stone for grounding the mind, helping someone to manifest their thoughts in a practical way. Its a good stone to work with if you find it difficult to focus or are a bit of a dreamer. It is fantastic for mental stress, and after holding it for a while it often feels like someone has given me a head massage! One day, I had it in my pocket and forgot about it whilst I was teaching a class. I became much more articulate and clearer at getting the teachings across, and the students seemed to able to grasp what I was saying. At the end of the day I rediscovered that little Sodalite in my pocket and then I knew why.
As always, with all the crystals mentioned, please don't take my word for it, but find your own truth. See how you feel when you carry round a certain protective crystal, and whether you feel stronger energetically by having it with you. You could try a few and see which one you prefer. Our energy field also changes all the time, so you may find you are attracted to work with different protective crystals at different times.