The bezels holding the stones are very narrow and thin. Sometimes a stone will pop out. If this happens, it could be just a matter of popping it back in the bezel. More often than not, the bezel was cut too long so the stone will be too loose in the bezel.
After popping the stone back in the bezel, the corner (arrow) as well as the area under the top ring showed light. Pop the stone out, place a very thin line of hypo cement in the bezel groove and pop the stone back in. Allow the glue to dry. Use a burnisher to push the bezel wire over the stone on both sides of the stone. Note: It really is not a good idea to sell a product with this fix. The ideal fix would be to remake the bezel, but that is a lot of work. Keep the stone and make yourself a pair of earrings. This happened on almost all the stones in similar short gemstone strands purchased from a different company at four times the price. I have 11 WireJewelry strands so far, and only this one stone popped out.