Which element is responsible for the red color in ruby and the green color in emerald?

The red color in ruby and the green color in emerald can be attributed to the presence of specific trace elements within their crystal structures. In the case of ruby, which is a variety of corundum, the vibrant red hue is primarily caused by chromium. This element replaces aluminum in the corundum structure, and the electronic transitions of chromium ions absorb certain wavelengths of light, resulting in the striking red coloration that rubies are known for.

Emerald, a variety of beryl, exhibits its rich green color due to the presence of chromium as well, although in some instances, vanadium can also contribute to the green hues. The mechanisms of color formation in both gemstones highlight how trace elements can significantly impact the final appearance of the stone.

Therefore, chromium is the correct answer, as it plays a pivotal role in imparting the notable colors of these two precious gemstones.