What is known as a gemstone's resistance to scratching?

The correct choice, hardness, refers specifically to a gemstone's resistance to scratching. This property is typically measured using the Mohs scale of mineral hardness, which ranks minerals from 1 (talc) to 10 (diamond). Hardness indicates how well a gemstone can withstand abrasion and is a critical factor in assessing its wearability and suitability for various types of jewelry.

Durability is a broader term that encompasses not just hardness but also other factors that affect the lifespan of a gemstone, such as the ability to withstand thermal shock or chemical damage. While durability is essential for gemstones, it does not specifically refer to resistance to scratching.

Toughness is the ability of a gemstone to resist breaking, chipping, or cracking when subjected to force. It is an important property but is distinct from hardness since toughness deals with the structural integrity of the material under stress rather than its scratch resistance.

Resilience is the ability of a gemstone to return to its original state after being deformed or stressed. This property is more relevant to how a gemstone behaves under physical pressure rather than its resistance to scratching.

Thus, hardness is the most precise term for describing a gemstone's ability to resist being scratched and is the reason why it stands as the correct answer.