Star Ruby  

 

I got two Star Rubies that I looted off of Hornets in Burning Woods withi a period of 2 weeks of intense hornet killing.

So you can safely assume it is ultra rare. In fact, during the time I got 2 star rubies I also got 4 black pearls and a sapphire.

So maybe it's a fragment of a percent of a chance for a gem to drop off of any mob above a certain level, say, level 40 (not sure of course).