This is the star of the Most Venerable Order of the Hospital of St John of Jerusalem. This particular example is the breast star of a knight or dame of grace of the Order. This example comes from my personal collection.
<a href="//commons.wikimedia.org/wiki/User:Evadb" title="User:Evadb">Evadb</a>
2008-10-20
<span class="int-own-work" lang="en">Own work</span>