Amber

Amber is exceptionally rare on Kytheria, owing to the fact that the world has not supported life nearly long enough for amber to form. All amber in the world either comes from the world of the gods or the very small handful of individuals who have amber as their Mark. Because of these associations, amber artifacts are always tied heavily to the gods and their worship. Carvings are rare because of how precious the substance is, and amber is instead polished and usually affixed to staves or scepters.
Taking ownership of amber artifacts is akin to claiming important standards in a battle; when religious organizations have conflict, theft or conquet and appropriation of amber often signals an important victory, and some religions have been effectively snuffed out by such coups. Destroying amber is immensely taboo regardless of one's faith; when Dramak the Sullen conquered the Stave Cathedral of an ancient kingdom in the middle continent, he burned the amber scepter of authority, and so incensed were the members of his own army that they turned on him and scattered his remains to the four corners of the world.
The amber appearing as one's Mark is small, as Marks tend to be, but does have all the properties of a Mark including retaining the memories of the person who had it and linking the bearer to the Girdle of the Gods. Amber Marks appear maybe once in a generation, if that, and fortunately are subject to numerous myths and fables about how taboo it is to kill the bearer of such a Mark (likely started by amber Marked themselves, but the results were successful). When worked into jewelry they're typically used in circlets or crowns, and can impart some of the original bearer's wisdom. They are also the easiest stone for one to make into a stolen Mark, which alone has been the subject of many an epic.