How about this:
\"In the shadowed depths below the lake Kadaikos lies a great black crystal. Legends say it contains the force that binds the people to the earth. The few who have seen it saw it gives off not light or color, it seems like it will even suck in any light around it. The even fewer who have touched it say it is the most dense object ever discovered and seems to be indestructible. \"
Since it is very dense it has a huge mass and thus lots of gravity. Science part solved.
I think it also fits into the world very well.
It would be located in the middle of Kadaikos (it would be deep under water in darkness so it\'s not easily visible), basicly an inverse of the Azure/Sapphire Crystal.
As the Azure/Sapphire Crystal represents life I think this black crystal can represent death.
Firstly it\'s shrouded in darkness.
There is a very good metafore for life and death, you can kind of see it in the map of the world. Water (which represents life) starts at the Azure/Sapphire Crystal (which represents giving life aka birth) and slowly flows and drips through the caves until it arrives at Kadaikos (which represents the collective lifes of the world) and the black crystal (which represents death).
It needs to be polished alittle, but I think it will work.