The Amethyst Box - Anna Katharine Green

On the evening before his marriage, Sinclair loses a precious curiosity from his collection: an amethyst box, containing a tiny flask of deadly poison. He suspects that this poison is in the possession of either his betrothed or her cousin, the girl his best friend Worthington loves. Turning to Worthington for help, they try to recover the box before the poison can be administered... (Summary by Carolin)