Final answer:
The main variables used to value gem corundum, which include rubies and sapphires, are color, clarity, carat, and cut, with color being particularly influenced by Cr3+ impurities in the crystal structure.
Step-by-step explanation:
When we describe the main variables used to value gem corundum, we're generally referring to rubies and sapphires, both of which are forms of corundum, a crystalline form of alumina (Al2O3). The value of these gems is largely determined by several critical factors. The first is color, which for rubies is a deep red due to the presence of Cr3+ impurities replacing Al3+ in the crystal lattice. Other important variables include clarity, carat (weight of the gem), and cut, as these affect the visual appeal and rarity of the stone.
For example, the presence of Cr3+ in corundum gives rubies their characteristic red color, an important aspect in determining its value. Clarity deals with the presence or absence of inclusions which can affect the stone's durability and light properties. Carat measures how large the gemstone is, typically with higher weight increasing the value. Lastly, the quality of the cut can greatly influence a gemstone's brilliance and overall aesthetic appeal.