That is the steel itself. Damascus is made by taking a single sheet of steel, folding it over on itself, and pounding it out dozens or hundreds of times. You end up with a block of steel that is cross sliced for each blade, and the pattern of those multiple layers is revealed when you give the blade an acid bath.
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Thanks for letting me know. It's very beautiful, almost mesmerizing to see the pattern across the blade. And it just look super cool!