Final answer:
Rubies are red because chromium (Cr3+) ions replace some aluminum ions in corundum, leading to absorption of green light. Sapphires are blue due to trace elements of iron and titanium. This demonstrates how the crystal lattice structure affects the color of gemstones.
Step-by-step explanation:
Rubies are red due to the presence of chromium (Cr3+) impurities within the corundum (Al2O3) lattice, while sapphires are blue because of trace amounts of iron and titanium. The substitution of a small percentage of aluminum ions (Al3+) with chromium ions in corundum results in the deep red color of rubies. On the other hand, sapphires owe their characteristic blue color to the presence of iron (Fe) and titanium (Ti) within the same type of lattice.
In the case of rubies, replacing Al3+ with Cr3+ in the corundum structure leads to rubies absorbing green light because the Cr-O distances are short, increasing the d orbital-ligand interactions and consequently allowing red light to be transmitted or reflected, which gives the gem its characteristic red color. Conversely, when replacing Al3+ with Cr3+ in beryl to create emeralds, the gemstone absorbs red light due to longer Cr-O distances, reflecting green light instead.