Sunday, October 24, 2010

Corundum (a.k.a. Sapphires and Rubies)

The minerals in the following pictures were soaked in vinegar, then scrubbed with water and baking soda.  The first picture of the vibrant blue sapphire has not been cleaned with anything.

Corundum is the mineral group that Sapphires and Rubies belong to.  A ruby is red corundum, and sapphire is every other color of gem quality corundum.  The blue variety of sapphire is called sapphire, while the other colors (except red!) are referred to as "fancy colored sapphires".  Light red and pink corundum are also sapphires, not rubies.  For industrial use, corundum is also used as an abrasive called emery.  

Parti-colored and Chatoyancy or Asterism
Because of their popularity and value, there have been many schemes involving enhancement treatment of the gemstone to get either more weight and/or better color quality from naturally poor quality gemstones.  I don't want to talk to much about that right now.  Sometime this winter I plan on publishing a post to this blog explaining some of the processes involved in enhancing corundum for the gem market, including the ethical heat treatment of gemstones, as well as such scams as laser drilling holes in diamonds and rubies, which are then filled with glass.  Buyer beware!

Most of the sapphires I've found are purple, or as a lot of people say, lavender.  Some are parti-colored (multi-colored), and I am certain that some are going to be star sapphires.   With only my 10x loupe, and without being able to polish them, I have been able to confirm the rutile needle inclusions that make the asterism (star) effect in one of the stones, however; I am certain more will display this after being polished.  I am also not ruling out the phenomenon known as "color change", meaning that in daylight, they are one color, and then under incandescent light they change a different color.  It is not common, but so far these rough stones are possible candidates.  

Black Star Sapphires

I have also recently found a spot with "black star sapphires".  If you do a search on the internet for "black star sapphire", you will see some awesome looking gemstones.  The "star", also known as "asterism" is caused by parallel rutile needle inclusions running in 3 different directions.  Black Star Sapphires can exhibit 6 and 12 ray stars, also referred to as "arms".

This one has really got me asking questions.  Is it a pseudomorph?  It also appears to have both pinkish purple and blue colors.