More information is needed to properly answer this question. The top 10 hardest firms in the world is too broad. Do you mean the hardest firms to get hired by, the hardest firms to work for, the hardest firms to sue?
diamond is the hardest thing on earth
the softest rock is talc and the hardest is a diamond
Silicon Dioxide (known as Quartz) is not the hardest mineral, the hardest mineral is an allotrope of carbon called diamond. Quartz is only the 7th hardest mineral.
The hardest naturally occurring substance is called diamond. It is a form of carbon.
aimillre
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Chiken
I like purple
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two U's day - Tuesday?
"400 is 20 squared"