Black Obsidian & Lapis Lazuli Crystal Necklace The Necklace in the picture is 22 inches long. Obsidian is an extremely protective gemstone. It will shield you against negativity and any ill will that is thrown your way. Clasp is made of a rare earth element called neodymium and is the strongest magnet clasp available. The Cord is a 100 pound test copolymer cord to insure there will be no fraying or breakage.