You can buy MOH's mineral sample kits from geology shops from $30-50 (so minerals from 1-talc up to 9-corundum) can be handy for determining other mineral's hardness to a closer range, if you have no samples with know hardness and could be looking at different hardness of minerals in the future.
I think there would be a chance(i may be wrong) that corundum would scratch another piece of corundum. So that would not prove its hardness ( one could be 8.99 the other 9.111 ?) trying the scratch test with a steel knife/nail can show if the mineral falls into a soft (5.5 and under) or hard (over 5.5) category, all the minerals you have should be over 5.5. From there no other rocks besides diamond and maybe other corundum should scratch the sapphire or ruby. The emerald being softer than both should not be able to mark either the ruby or sapphire, but itself should be marked by both.
Really wish i could help you more but my knowledge is very limited. I hope they all turn out genuine.
I think there would be a chance(i may be wrong) that corundum would scratch another piece of corundum. So that would not prove its hardness ( one could be 8.99 the other 9.111 ?) trying the scratch test with a steel knife/nail can show if the mineral falls into a soft (5.5 and under) or hard (over 5.5) category, all the minerals you have should be over 5.5. From there no other rocks besides diamond and maybe other corundum should scratch the sapphire or ruby. The emerald being softer than both should not be able to mark either the ruby or sapphire, but itself should be marked by both.
Really wish i could help you more but my knowledge is very limited. I hope they all turn out genuine.
