Cordierite is a rock forming mineral that simply does not generally give us beautiful specimens, let alone translucent to transparent gem crystals. Iolite is the variety that does, sometimes... and this is from a very old find in Brazil (not sure how old, but I know it predates the 1970s from Larry Conklin's stories on his large famous specimen — sold long ago). In my career of 4 decades as a mineral dealer, I have seen only a handful of these turn up in fine crystals, of which this is one.