The correct answer is: D. Diamond
A diamond is a chemical element with the symbol C and the atomic number 6. It is a member of the carbon group in the periodic table, and is the second-most abundant element in the Earth’s crust, after oxygen. Diamond is the hardest naturally occurring substance known, and is widely used as an abrasive, as well as in jewelry.
Ruby and sapphire are both gemstones that are varieties of the mineral corundum. Corundum is an aluminum oxide mineral with the chemical formula Al2O3. It is a hard, brittle, and transparent mineral that is typically red, blue, or green in color. Ruby is a red variety of corundum, while sapphire is a blue variety of corundum.
Emerald is a green variety of the mineral beryl. Beryl is a beryllium aluminum cyclosilicate mineral with the chemical formula Be3Al2Si6O18. It is a hard, brittle, and transparent mineral that is typically green, blue, or yellow in color. Emerald is a green variety of beryl.
In conclusion, the only element among the given options is diamond.