Just bought this "Mali sandstone" necklace which I think is just gorgeous! But has anyone worked with this stone before? It "sheds" a powdery residue when you wear it. I washed them and that helped some. The largest beads are very big--almost 40mm. Though it isn't old the beads are drilled from both ends which is kind of neat. Any help on how to stabilize the material so it can be worn would be helpful. Steve