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The Chemistry Behind the Colours of Rubies and Emeralds
Context:
Inorganic chemistry explores the chemistry of all elements in the periodic table, with a major focus on transition metals.
Transition metals are elements found in the d-block of the periodic table, typically within groups 3 to 12. The International Union of Pure and Applied Chemistry (IUPAC) defines a transition metal as an element with a partially filled d subshell or one that can form stable ions with an incomplete d subshell.
Role of Transition Metals in Colour
- Transition metals, including iron (Fe) and gold (Au), are known for their intense colours. Many natural substances, such as gemstones, pigments, and even haemoglobin in blood, owe their colours to transition metals.
- Transition metal compounds often display vibrant colours due to their electronic structures.
- Rubies and emeralds are prime examples of how a small amount of chromium can create stunning colours in otherwise colourless minerals.
Crystal Field Theory: Explaining Gemstone Colours
- Developed by physicists in the 1920s to explain how transition metal ions interact with surrounding atoms.
- Helps predict how a transition metal ion’s properties change based on its surrounding elements.
- Rubies: Composed mainly of corundum (Al₂O₃), where aluminium ions are surrounded by six oxygen ions.
- Emeralds: Composed mainly of beryl (Be₃Al₂Si₆O₁₈), with a more complex crystal structure due to additional elements like beryllium and silicon.
- Pure corundum and beryl are colourless; their colours arise from the presence of chromium.
How Chromium Creates Colour?
- Rubies: Chromium replaces ~1% of aluminium in corundum, absorbing blue and green light, and making rubies appear red.
- Emeralds: Chromium replaces aluminium in beryl but interacts more weakly with surrounding oxygen due to the presence of silicon and beryllium. This weaker interaction causes emeralds to absorb blue and red light, making them appear green.
The Science of Colour Perception
- Sunlight consists of all visible colours (ROY G BIV: red, orange, yellow, green, blue, indigo, violet).
- Objects appear coloured because they absorb certain wavelengths and reflect others.
- If blue is absorbed, an object looks red.
- If red is absorbed, an object looks green or blue.
Implications
- Understanding how transition metals interact with their surroundings is crucial in designing chemical compounds for specific purposes.
- This research helps in understanding both the basic science and practical applications of metal-containing compounds.
Understanding Minerals, Crystals and Gemstones
- Minerals are naturally occurring, inorganic solids with a definite chemical composition. Minerals have specific physical properties (colour, hardness, cleavage, and crystal habit) that help in their identification.
- Examples: Quartz (amethyst, citrine), diamond, corundum (ruby, sapphire), beryl (emerald, aquamarine), topaz, and more.
- Crystals are solid materials whose atoms are arranged in a highly ordered, repeating pattern. This regular arrangement of atoms gives crystals their characteristic shapes and physical properties.
- Examples: Quartz (amethyst, citrine), salt, snowflakes, and sugar crystals.
- Gemstones are minerals, stones, or organic materials cut and polished for beauty, durability, and use in jewellery or other adornments. They are valued for their rarity, colour, brilliance, and sometimes special optical effects (e.g., chatoyancy or asterism). Most gemstones are minerals, but some, like pearls and amber, are organic.
