An alexandrite is a form of chrysoberyl which displays a colour change dependent on the light source, with strong pleochroism.

Sea sediment is mud found at the bottom of an ocean. "Iolite" is another name for the mineral cordierite, a magnesium iron aluminum silicate, which is noted for unusually strong pleochroism. Transparent cordierite makes beautiful gemstones, in shades of blue.

Locally one can find tanzanite jewelry at JCPenney. Tanzanite is a stone that is becoming more difficult to find. But one can also purchase tanzanite jewelry online at Gemvara and Amazon.

Opal is a mineraloid gel. It is made, oddly enough, almost entirely of opal.

It is also completely free of birefringence, and has virtually no pleochroism.

Opal is basically silica. Saying opal is made of opal is like saying quartz is made of quartz, or tiger eye is made of tiger eye. It's the unique structure of the silica that makes it become opal. The Japanese have made a nice man made opal using silica and zirconium oxide.

Si02nH20