[78-L] Glass Compound 78s

[78records at cdbpdx.com]

Greetings.  I am wondering how to easily identify glass compound 78s.  I was playing a record from 1942 and I noticed the sunlight was sparkling off the record in a rainbow of color, as if the light was being reflected through a prism.  I examined the record closely and discovered I could see brightly lit images through it.  It has an unfortunate crack and the edges of the crack seem to be flaking off like glass.  

Could this be a glass compound record?  Are there any sure-fire ways to identify glass compound records besides cracking them and looking to see if the edges are flaking off?  

Thanks!  CDB