Resistance of the stone to scratch. The harder the stone the harder it is to scratch, and may be scratched only by stones of the same degree of hardness or higher. A Diamond for example, can only be scratched or polished by another Diamond. In order to polish Sapphire or Ruby one would need a Diamond or corundum drill

The Moha chart of hardness listed by Moha, was the first to examine and list the hardness of stones