vicke
If there is arsenic in it it will give off a garlic oder when you strike the two pieces togeather, like trying to create a spark with flint. If it does put in a well marked box and wash your hands.
It sounds like it has a hardness of 7.5 which would make it harder than the average arsenic bearing rock. Try your hardness test the other way around. See what leaves a scratch on your rock. The steel, glass, flint, and saphire are all you need to test with. Get a magnifying glass and look to see if there is a scratch or did the scratching device just leave a trail of powder.