Silver-plated brass. They are not 15mm long, they are very small - no longer than 7mm. I took one out to evaluate the quality and it doesn't screw on properly to close it. If you unscrew them to open them, don't expect the three ribs inside to keep them closed afterwards. I've seen better quality jewelry clasps for literally a dollar. Not impressed and because I don't live near a UPS store and Amazon wants to charge me more than the item's value to have UPS pick it up for a refund I'll smash them and take a loss.
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