Final answer:
A yellow gem composed of aluminum oxide with a hardness of 9 on Moh's scale and which crystallizes in the hexagonal system, is known as yellow sapphire.
Step-by-step explanation:
A yellow gem with a composition of aluminum oxide, hardness of 9 on Moh's scale, and belonging to the hexagonal crystal system is known as sapphire. Sapphires are gem-quality crystals that can come in various colors, with the presence of different impurities determining their hue. While blue is the most commonly known color for sapphires, due to impurities such as iron and titanium, yellow sapphires are indeed a natural variant. It is the aluminum oxide compound that gives sapphires, including the yellow variant, their hardness, which is second only to diamonds in the natural world.
Corundum is the mineral name for aluminum oxide and variety of colors in sapphires are due to the presence of various impurities within the aluminum oxide lattice. When the impurity is chromium, the gemstone is known as a ruby, but for a yellow sapphire, the color can be due to the presence of trace amounts of iron. The high hardness and the fact that they belong to the hexagonal crystal system further confirm that the yellow gem in question is a sapphire.