The Vikings probably mined iolite from deposits in Norway and Greenland. Viking sailors used iolite as a polarizing filter to find the sun on cloudy days for a safe offshore navigation. Iolite is usually a purplish blue when cut properly
How to Use and Buy Polarizing Filters - Digital Photography School samfeinstein Be sure to get a "circular" polarizer (for TTL metering) and if you want it on a wide angle lens, get s "slim" filter that won't cause vignetting.
* How to Use and Buy Polarizing Filters - great for shooting in bright sun to get more vibrant color to skies and landscapes. Hoya is a good brand. Take note of the diameter of your lens before making a purchase as there is a large variety of lens sizes.