According to legend, Vikings used iolite slices to reduce glare when checking the sun’s position.

When tanzanite was first discovered, gemmologists initially mistook it for cordierite, a long-known transparent, pleochroic gem with a violet-blue hue. Today, cordierite is better known by its trade name, iolite, derived from the Greek word "ios," meaning "violet."

Iolite is renowned for its strong pleochroism, which makes it challenging to cut to achieve the best colour. This characteristic presents ongoing difficulties for producers and buyers who aim to market this alluring yet complex gem as an affordable alternative to blue gemstones.

This aluminium, iron, and magnesium silicate displays two main features: a captivating violetish-blue to slightly violetish-blue hue, imparted by iron, and notable pleochroism. Depending on the angle, iolite can show different colours—violet ones exhibit shades from light violet and dark violet to yellow-brown, while bluish iolites reveal colours ranging from colourless to yellow, blue-grey, and dark violet. Consequently, a bluish Iolite might appear colourless or yellow from some perspectives, while a violetish one could look brown.

Iolites are predominantly cut into faceted gems, though cabochons are also common. They typically range from 1 to 10 carats, with gems over 5 carats being particularly rare.

On the Mohs hardness scale, Iolite scores between 7 and 7.5. However, due to its cleavage in one direction, its toughness is considered fair, making it susceptible to breakage if set in rings or other pieces exposed to daily wear.

Unlike Tanzanite, Iolite is seldom treated. Its striking blues and violets are naturally occurring, which is a notable advantage compared to other blue gems like blue Topaz and Sapphire, which often undergo various treatments.

Not all Iolites exhibit fine colour; some can appear greyish or nearly colourless. Unfortunately, the chemistry of iolite precludes the possibility of treatment to enhance these less desirable colours. Interestingly, the deep blues of some Iolites are thought to be caused by an iron-titanium charge transfer, similar to the blue in Sapphire, but unlike Sapphire, Iolite cannot be heat-treated to boost its blue colour due to its lower melting point.

Iolite, being relatively hard, is commonly found in alluvial deposits. Besides Sri Lanka, it is sourced from several regions in Africa, including Kenya and central Tanzania, as well as India, Brazil, and Norway. A significant deposit was also discovered in Madagascar in 1994.

Despite its beauty, Iolite has not gained the same popularity with retailers as Tanzanite did in the 1970s and 1980s. Experts suggest that the inconsistent quality and supply of Iolite have prevented jewellery designers and retailers from fully embracing it for their collections.