I is for Iolite
Iolite comes from the mineral cordierite, a silicate mineral found in igneous and metamorphic rocks. When the mineral is transparent and of a higher quality, gemologists refer to it as iolite.
Iolite is a stone that imposes some challenges when faceting because of it’s pleochroism (the effect of seeing different colors when viewed from different angles). The lapidary artist must examine the stone to determine the proper plane on which to cut the facets. Otherwise, the cut stone will not showcase the gorgeous violet color it is known for. Instead, the stone might reflect little color.
The mineral cordierite is used commercially in catalytic converters although in most cases, a synthetic form of the mineral is used. This allows for consistency in both supply and quality. Cordierite allows for rapid heating and cooling. (You may have heard about the theft of catalytic converters from cars. The thefts are because of the scarcity and value of the precious metal rhodium.)
Cordierite is also used in applications where reduction of thermal shock is important – like the pizza stone in your kitchen or the shelves in a ceramic kiln.
Iolite that has a rich hue, can look very much like tanzanite. Grading and quality of the stone is key. It is a reasonably hard stone at 7-7.5 on the Mohs scale. The name iolite comes from the Greek word for violet and is sometimes used as a substitute (and less expensive) stone in place of a sapphire.
I have my eye on a hexagon faceted iolite for a future jewelry project and at the low price, I can afford it! You can see the stone here.
This iolite below is the largest cordierite (63.83 carats) gemstone in the National Gem Collection at the Smithsonian.