The brittleness might be a problem in ants, since it would be hard to shield; might work as sacrificial armour, perhaps.

Back to metals, Rhodium (not sure I trust my spelling at all today - is that right?) is very hard - white gold gets coated with the stuff. I believe its the hardest pure metal (so Im told), which probably makes it useful for plating things, but I dont know how it compares to strange alloys. Plus its a tad pricey.

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Fluppet