The Amethyst Box

On the night before his wedding, Sinclair finds that his precious amethyst box containing a tiny flask of deadly poison is missing. And he knows it can only have been taken by either of two people: his soon-to-be wife or her cousin. Together with his friend Mr. Worthington, he must find who stole the poison and stop them before they use it. 'The Amethyst Box' (1905) is a short story by American crime writer Anna Katharine Green; a suspenseful "whodunit" with a dash of romance, which concludes thoroughly unexpectedly. Anna Katharine Green (1846-1935) was an American poet and novelist.
She is best known for her novel 'The Leavenworth Case' (1878) with which she became one of the first American writers of detective fiction. Agatha Christie cited Green as an influence on her own writing.