Pleochroism

I love to observe optical properties in gems and minerals and one of my favorites is Pleochroism.
Pleochroism is the ability of a stone to display two or more colors when viewed from different angles. Some good examples are Kunzite, Tanzanite and Cordierite
Good cutters know exactly how to orient the stone to be viewed from the best angle. Like for example a  light pink Kunzite shows a much stronger pink color when viewed from the c-axis. So the best way to cut it, is to put the table of the stone where the c-axis is.  Dichroism is basically the same thing, although a dichroic stone only shows two colors while a Pleochroic can show more than two.  I came across a small lot of Cordierites that show perfectly the pleochroism on Cordierites (Iolites):

If you were to facet a piece of rough like this, you could choose the color of the stone from yellowish to deep blue. What color would you choose?

Emerald Trapiches

Emerald is a green variety of Beryl. The color is caused by Chrome or Vanadium. Depending on the color and clarity the price could range from a few dollars to many thousands per carat. Emeralds are found in every continent and the most prized ones are from Colombia.
Usually flaws decrease the price of the stones, not unlike most gems. One exception is the trapiche.
Trapiches are a rare kind of Emerald with the presence of black carbon impurities giving the Emerald a radial six pointed star pattern. It is usually made into oval and round cabochons but I have seen different shapes too. More recently, Colombia has produced some different trapiches. They look more like flowers. The carbon is thicker with sharper definition. It looks best when sliced, like the watermelon Tourmalines. Sometimes the center is black but it can also have a green center. Each piece is unique and the contrast of green and black is quite beautiful.