Just an old mirror. Why it is said to be a memento will never be known.
The Garai Keepsake is a mirror once kept by Garai and considered a memento. Radius kept it with him after Garai's death, as it holds the power to infiltrate the inner areas of the Isle of the Damned. As its description reads, "why it is considered a memento by Garai will never be known."
From: Key Items (Chrono Cross)