Amethyst

The stone is set in intricately wrought metal like silver, but more lustrous. It doesn't appear to be tarnished at all with age, but you get the sense that you are holding something very, very ancient. The swirling design around the top is pretty, but when you look closer you notice that tiny pictures are part of the pattern, somewhat like celtic illuminated manuscripts. You can make out a mermaid, a wolf or dog of some sort, and a rose, but the rest would need a magnifying glass. Just then you hear the front door downstairs open. With a guilty start, you drop the locket back into the chest, slam the lid shut, and hurry downstairs to greet your grandmother.