I suggest the gold-plated glossy material. The pitting and smoothing of the model that occurs when the plating is applied makes the key look old and worn. It's fantastic.
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Billina showed her the place where she had "stubbed her bill," as she expressed it, and Dorothy dug away the sand until she felt something hard. Then, thrusting in her hand, she pulled the thing out, and discovered it to be a large sized golden key--rather old, but still bright and of perfect shape.

"What did I tell you?" cried the hen, with a cackle of triumph. "Can I tell metal when I bump into it, or is the thing a rock?"

"It's metal, sure enough," answered the child, gazing thoughtfully at the curious thing she had found. "I think it is pure gold, and it must have lain hidden in the sand for a long time. How do you suppose it came there, Billina? And what do you suppose this mysterious key unlocks?"

From Ozma of Oz by L. Frank Baum
Full text available at http://www.gutenberg.org/etext/486

[Emerald not included.]