I wrote to Howard at www.theperfectedge.com and he said that it was hard to tell from the pictures but if the stone was greener than the picture and very smooth, it could be a yellow-green Escher. When this stone is side-by-side with my coticule bout, you can see that it's a different stone. It's greenish and much smoother than the coticule. I think it's an Escher.
If it's an Escher, does anyone have an idea of what it's worth?