116 PROCEEDINGS OF THE ACADEMY OF [Feb., 



almost as sharp as those from opal, proving that the colors are due to 

 the repeated parallel laminae and not to the lines or corrugations, 

 although the latter may be the only visible indications of structure 

 when viewed with the microscope in the usual manner. 



While they probably know or care little about optical science, the 

 fabricators of false gems have not failed to note the resemblance 

 between the colors of pearl shell and those of opal, and an imitation 

 of opal used in cheap jewelry is made by cementing a thin section of 

 pearl shell between two rounded and polished pieces of glass. 



