From around 1000 AD, it is a huge disc cross with Celtic interlace and plaitwork patterns, figurative scenes including a hunting scene, and inscriptions telling us who made it and who erected it.There are 17 early Christian stones, plus 11 memorials and other stones from the post-Norman periods.
It is a stone pillar, 1.01 metres (3.3 ft) high, with four lines of Latin inscription and an incised cross.
On the back there is an Ordnance Survey bench mark, and various more recent carvings. It originally stood on a cairn on Margam Mountain, between Bridgend and Maesteg.
The Ordnance Survey cut a bench mark on it, and graffiti was also carved onto the stone, before it was moved to the museum, with a replica replacing it at its original location These date from 600 to 900 AD and are crudely produced compared to the later sculptured crosses.
Other stones from the Abbey and the local area were added to the collection, which is now in the care of Cadw.
Of the 30 or so ancient carved stones in the museum, 17 are pre-Norman, and are displayed on the ground floor.