Final answer:
Ruby and emerald have different colors because chromium ions interact with different chemical environments in corundum and beryl, leading to red in ruby and green in emerald.
Step-by-step explanation:
The difference in color between ruby and emerald, despite both containing chromium (Cr³+), can be attributed to the distinct chemical environments in these two minerals. When Cr³+ replaces Al³+ in corundum, the surrounding oxygen atoms and the crystal field they create cause the absorption of light in such a way that red light is transmitted, giving rubies their characteristic hue.
Conversely, in beryl, the presence of additional elements such as vanadium and occasionally iron, together with the unique structure of the host mineral, leads to different absorption of light wavelengths, resulting in the emanation of green light, which is why emeralds are green.