Final answer:
The three main mechanisms of color generation in gems are: the presence of trace impurities (e.g., Cr³+ in rubies and emeralds), structural defects and inclusions (as seen in some colored diamonds), and dispersion causing diamonds to sparkle. The host lattice and chromophore environment significantly influence the perceived color of the gemstone.
Step-by-step explanation:
The three main mechanisms of color generation in gems are due to trace impurities, structural defects, and dispersion. Gem-quality crystals such as ruby and emerald derive their colors primarily from small amounts of Cr³+ impurities in an otherwise colorless metal oxide lattice, gifting ruby with its famous red and emerald with its distinctive green hues. Rubies are considered allochromatic minerals, gaining their color from impurities, whereas idiochromatic minerals get their color from essential components.
In both rubies and emeralds, the environment of the chromophore, Cr³+, varies due to different host lattices, which in turn affects the colors we perceive. For instance, rubies have a Al2O3 host lattice causing shorter Cr-O distances, resulting in the absorption of green light and transmission of red. In contrast, emeralds have a Be3Al2Si6O18 lattice with longer Cr-O distances due to [Si6O18]¹²- silicate rings, causing them to absorb red light and transmit green.
Furthermore, dispersion and internal refraction contribute significantly to the sparkle and brilliance seen in diamonds. This phenomenon occurs because of a high index of refraction and critical angles that encourage multiple internal reflections, leading to bright, sparkling exits of light. Structural defects and inclusions of other materials, such as graphite, can also give rise to different colors in gems.