I love iolite too. And I have to say, Uruguayan amethyst is so deeply purple it's such a joy to meditate with.
Regarding your question, there are some
Third Eye Chakra crystals of course. Personally, I recommend labradorite and lapis lazuli. Even high quality specimens can be relatively cheap. Aside from the two I previously mentioned, I also particularly like dumortierite, the stone of Tarot, especially those that are mostly deep blue-violet. I find them particularly effective and I use them when I conduct some very important readings.