The smaller jewel on the left is a replica of the Hampden Jewel, which is said to have been worn around his neck by John Hampden at the Battle of Chalgrove. The original (right), which is a cornelian in a silver setting, was given to the Bodleian Library by Thomas Knight in 1795, and has engraved around the edge the couplet, 'Against my King I do not fight, But for my King & Kingdom's right.'
The Bodleian Library has produced these replicas, which are on sale in the Library shop or via the website. To order, go to the Bodleian Library Shop. The replicas do not have the couplet engraved around the edge, but a copy is included in the presentation box.