RORY: Where is she [her great-grandmother] now?
EMILY: In the living room, scratching the diamond pendant I bought her against a mirror.
A common technique in the past to test if a diamond was real or fake was to rub it against glass – if it made a scratch or scrape mark on the glass leaving the jewel untouched, then the diamond was genuine.
Many modern artificial diamonds can actually pass this test, and if you’re unlucky, it is possible to damage a real diamond rubbing it against glass, so it isn’t really helpful. It is probably going to ruin Emily and Richard’s mirror, so it’s inherently selfish as well as rude.
The Annotated Gilmore Girls 