Chippewa Valley Times

The striking colors of rubies and emeralds, which define the shades of red and green respectively, have long fascinated both scientists and enthusiasts. These vibrant hues are the result of complex interactions at the atomic level involving a common transition metal: chromium.

An inorganic chemist explains that the field focuses on understanding the chemistry of elements, particularly transition metals like iron and gold. These metals are known for their intense colors in compounds, as seen in nature through gemstones and pigments.

Both rubies and emeralds are minerals with consistent chemical compositions and highly ordered structures. When these structures extend in three dimensions, they form crystals. Crystal field theory, developed by physicists in the 1920s, helps explain why these gemstones have their distinct colors. This theory predicts how a transition metal ion's structure is influenced by surrounding atoms.

Rubies consist mainly of corundum, made from aluminum and oxygen. In contrast, emeralds primarily comprise beryl, containing beryllium, aluminum, silicon, and oxygen. Pure forms of these minerals are colorless; however, small amounts of chromium replace about 1% of aluminum during formation under high temperature and pressure conditions underground.

The presence of chromium is key to the color differences between rubies and emeralds. Chromium ions absorb blue and green light when surrounded by oxygen ions in rubies, reflecting red light back to the observer. In emeralds, silicon and beryllium weaken the interaction between chromium and oxygen ions. This causes absorption of blue and red light, leaving green as the visible color.

This ability to alter properties through changes in surrounding elements is central to inorganic chemistry research. It aids scientists in understanding metal-containing compounds' basic science while designing chemicals for specific uses.

The article highlights how chemistry reveals nature's method for creating gemstone colors using various complex structures formed with periodic table elements.