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Though not approved by the US Food and Drug Administration (FDA) for use on humans, obsidian is used by some surgeons for scalpel blades, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, the cutting edge of the blade being only about 3 nanometers thick.[35] Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope; when examined even under an electron microscope an obsidian blade is still smooth and even.