The amethyst one has the strongest protective effect, especially when the merkaba - three-dimensional Star of David - is spun (very helpful for turbulence on a plane). My only criticism is that the "cage" around the spinning merkaba (in the middle) is a little tighter than it should be, so the merkaba doesn't spin easily - hence four stars only. Also, if you wear it as a pendant (for protection), the merkaba occasionally falls out.
The Gem Tree service was prompt and polite, especially when I ordered another one but they had just sold their last one. Also, they have kept the price down, when other retailers are demanding £30 or even £40 for this!