Man made items with a minimum hardness of 7?

Hi,

I have been searching streams and access roads for good natural materials to use with my steel as the flint won't last forever.

Yesterday on my hike, I picked up a bunch of rocks and threw them into my rock bag for later identification/testing. One of these rocks I picked up turned out to be a piece of porcelain. The pocelain was hard to identify, because only one side showed a glaze (which made it hard to identify at first when trying to identify it as a rock).

Anyway, It got me wondering...

What other man made materials (readily available) are hard enough to generate sparks from a steel?

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Last edited by SDC; Today at 01:09 PM.. Reason: spelling

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Man made items with a minimum hardness of 7?