Also known as cordierite, Iolite, is believed to be a Vikings Compass Stone. They used it to reduce the sun brightness or to locate the sun in cloudy days, which helped them to understand their location in the sea. When tanzanite was first found, miners confused it to already well known iolite. Its violetish Blue color could be confusing for gemstone merchants.
Due to the very strong yellow and colorless pleochroism, it is also a challenge for cutter to choose the right direction for cutting. It's very tricky - cutters mistake can make the stone look yellowish/colorless, which reduces its value