Obsidian Byzantine Cross etched with 8 pointed star and oak leaves

I live near the Roman Wall World Heritage Site, one day while digging in the garden next to the plum tree, I discovered a black cross. At first I thought it was made of black glass, having shown it to a few people they think it is obsidian. When held up to the sunlight it shows through as bright pinky violet.

The holes at the top of the cross do not meet, which leads me to believe the loop had open ends gripping the cross.
The 8 pointed star shows lines for the 4 corners of the earth, and the solstices and equinoxes.
The oak leaves are etched on the top the sides and 2 on the bottom longer part of the shank.