At its best, Iolite is a beautiful shade of deep violet-blue, and as blue stones are fairly rare, it's a good option as it remains affordable.  It is also trichroic; that is, you can see different colours when you view it from different angles; dark violet-blue, pale greyish-blue and yellowish.  Iolite can be a washed-out mauve colour, but the stones I have are an intense shade of indigo, and - as cabochon grade iolite often is - a little included, but due to the depth of colour, this is not at all obvious.