I recently picked up a slightly unusual Escher.
A 5" x 2" main stone with a slurry stone. Nothing unusual there. The main stone is grey. Again, nothing unusual. At first glance the slurry stone was dull yellow/green. Now, that's unusual. Yellow/green main stone with grey slurry stone wouldn't have surprised me.
On closer inspection the slurry stone was a dull yellow green on one side and a more grey colour on the other. Running straight through its centre is a sedimentary band deliniating the yellow/green from the grey.
This shows that the grey and the yellow green are found in the same deposit. However, they are somewhat different in hardness and 'feel'.
This is much like the blue coticule and yellow coticule being found in the same deposit except that the difference between the grey Escher and the yellow/green is nowhere near as marked as the difference between blue and yellow coticule.
IMHO, the yellow/green is the better stone.
I'll take more images when I can get the light right. In the interim, there's a sample image and two videos illustrating the slurry stone received.
http://www.upaboveit.com/images/misc.../eRIMG0504.jpg
Video:
Video 1 - 8.06MB
Video 2 - 7.99MB
Probably best to right click and save locally before viewing.