Colored Trays (the base holding the glass image) can often have flaws on the back side due to the color coating process done in the factory.
Using Sharpie Markers, Nail polish, Acrylic paint can hide the flaws such as chips and scratching flaws on Cabochon base.
If you read Amazon reviews, the buyers often complain because this flaw is silent and never listed in most descriptions. The more the Buyer knows, the BETTER.
Hope this little tip is helpful