Gemology Hub | Gemstone & Mineral Identification Guides

A gemstone can look different under two lamps without necessarily qualifying as alexandrite. Lighting changes the colours available to illuminate a stone. A narrow-spectrum lamp can therefore create a striking visual shift that needs careful interpretation. What makes alexandrite different? Alexandrite is a variety of chrysoberyl associated with a definite colour change caused by chromium. SSEF’s 2012 news slides compare this with a chrysoberyl that looked different under a particular LED but showed almost no main colour change between daylight and incandescent light. Why the comparison conditions matter A specialised lamp can emphasise only part of a gemstone’s response. The appearance under that lamp is interesting, but it should not replace a comparison under standard lighting conditions. The same caution applies to photographs whose lighting has not been explained. SSEF’s later public article reinforces the need to consider an appropriate colour change together with chromium-rela...

Sapphire is often associated with blue, but the corundum family offers a much broader colour range. Yellow, pink, violet and green sapphires are examples of the fancy colours discussed in SSEF’s 2012 Facette. A colourless ideal composition can become colourful through very small changes. What produces the colours? Pure corundum is aluminium oxide. Small amounts of other elements can enter its crystal structure and change which wavelengths it absorbs. Chromium contributes to red ruby colour, while iron and titanium are important in many blue sapphires. The colour depends on more than the total amount of one element. Its chemical state, combinations with other elements and colour centres in the crystal can all matter. Some corundum also shows different colours or shades in different viewing directions, a property called pleochroism. Why colour names have boundaries SSEF discusses the difficulty of separating nearby colour categories, such as pink sapphire and ruby. Variety names i...

Spinel is more than a red gemstone. Natural examples can be pink, violet, blue and many other shades. Its colour range reflects small differences in chemistry, while its crystal shape and geological setting add further interest for collectors. The role of colour-producing elements SSEF’s 2011 overview describes chemically pure spinel as colourless. Chromium can contribute pink and red, while iron, vanadium and occasionally cobalt influence other colours. The mix and amounts matter, so there is no single colour for all spinel. The article discusses classic and newer sources, including Myanmar, Sri Lanka, Tajikistan, Vietnam, Madagascar and Tanzania. These are historical source examples, not a complete list of every occurrence. Crystal shape is another clue Spinel has a cubic crystal structure and commonly forms octahedra, which resemble two pyramids joined at their bases. Twinning can create interesting shapes. A rough crystal can therefore be attractive even before it is faceted...