The colour of a gemstone is not a sufficient basis for identification. A red stone could be a ruby, a green stone could be an emerald, and a colourless stone could be a diamond, but colour is only the first step in identifying a gemstone. It is not a reliable identification. Natural gemstones, lab-grown stones, glass substitutes, and stones that have been dyed or treated often have very similar colours. The colour may appear differently depending on the lighting, the background, surface treatments, or the depth of the stone.
The first step in examining a stone is to observe it on a neutral background under a source of diffuse white light. Wipe the stone clean with a lint-free cloth, and use gem tweezers or a stone holder to avoid fingerprint smudges. Note the bodycolour, the intensity of the colour, and whether the colour is even or uneven. Then turn the stone slowly, looking for zones of colour, dark spots, paler streaks, or colours seen from only some angles. This information is much more helpful than simply stating that the stone is “blue” or “green.”
The second step is to look beyond colour. Note whether the stone is transparent, translucent, or opaque, and note the type of lustre and any condition of the surface. Glass may show rounded facet edges, abrasions, or bubbles. A gemstone will show different internal and surface characteristics. No single observation can be considered definitive; the significance of each observation lies in how well it agrees with other observations to suggest a particular identification.
A 10x magnifying lens allows you to see more detail about a gemstone than with the naked eye. Place the lens close to your eye, and move the stone closer to the lens until it is in focus. Use a directional light and rotate the stone to examine it from all angles. Look for cleavages, fractures, crystals, scratches, abrasions, and surface-reaching features. Reflections will move as the stone rotates, but an inclusion will remain stationary within the stone. You may also notice dust particles on the surface of the stone, so check again after you have cleaned the stone.
To practice, select two stones of a similar colour. They do not have to be gemstones. Use two mineral samples, synthetic gemstones, or pieces of coloured glass. Describe each stone without identifying it. Note the transparency, the lustre, the quality of the facet edges, the internal features, and any visible zones. After completing your descriptions, consult your reference material. What materials might these stones be? This exercise is designed to train you to look at objective evidence before allowing your expectations to guide what you see.
Colour can help in identifying a stone when used in combination with other observations and measurements, such as the refractive index, birefringence, specific gravity, fluorescence, or pleochroism. Although the use of simple equipment helps to eliminate many possible gem materials, each test has limitations. The refractometer may show no reading if there is poor contact between the stone and the glass, or the stone may be too deeply mounted to allow the instrument to touch it, or a stone may have been treated in a way that alters its optical properties. Your notes should clearly distinguish what you have observed, what the instrument readings were, and what you have inferred from those readings.
Instead of asking, “What colour is this stone?”, ask yourself, “What set of properties does this stone have, and which possible identifications can these properties eliminate?” Your skill improves as you learn to record precise observations, to let go of preconceptions, and to justify why you need additional tests rather than guessing a gem material based solely on its colour.
