Color Change and Play of Color, Explained Simply
Alexandrite shifts color as the light source changes, opal diffracts light into flashes, and labradorite reflects it from inside.
Three stones that are famous for changing appearance do it for three unrelated physical reasons, and knowing which mechanism is at work tells you how to check the stone.
Alexandrite: a shift in absorption
Alexandrite, a variety of chrysoberyl colored by chromium, transmits light in two narrow windows, one in the blue-green and one in the red. Which window dominates depends on the light falling on the stone. Daylight is rich in blue and green wavelengths, so the stone looks green or teal. Incandescent light is rich in red and yellow, so the same stone looks red to purple.
The color change is a real change in what the stone transmits under different illumination, and it requires two different light sources to demonstrate. Under a single lamp, a color-change stone and an ordinary stone can look identical.
Related effects exist in color-change garnet and color-change sapphire, and synthetic color-change corundum is made as well. Two things to keep separate: pleochroism, which is a stone showing different colors when viewed from different directions under the same light, as tanzanite and iolite do, and color change, which requires a change of light source.
Opal: diffraction
Precious opal is built from submicroscopic spheres of silica stacked in a regular three-dimensional array. Light entering that array is diffracted, split into its component wavelengths, and sent back out at different angles. The spacing of the spheres sets which colors appear, and the angle of view sets which ones you see at any moment.
That is why opal's flashes move as the stone moves. It is also why the effect needs a cabochon: a flat, polished surface shows the pattern across the whole face.
Not every opal does this. Common opal has no such internal array and shows no play-of-color. Fire opal has a strong orange to red body color and may or may not flash. Boulder opal is opal formed in ironstone, which gives it a natural dark backing that makes the flashes read strongly.
Lab-grown opal exists and tends to show very regular, evenly sized patches of color arranged in columns, where natural opal's patches are irregular. Regularity is a clue, not a verdict, and a laboratory settles the question.
Labradorite: interference from within
Labradorite is a feldspar that separated into extremely thin alternating layers as it cooled, each layer slightly different in composition. Light reflecting off those stacked layers interferes, and the interference cancels some wavelengths and reinforces others, producing flashes of blue, green, gold, or copper that appear and disappear as the stone turns. The effect is called labradorescence, and it comes from just inside the surface rather than from the body of the stone.
Feldspars produce a family of related effects. Moonstone shows adularescence, a soft floating sheen rather than sharp flashes, produced by light scattering within its layered structure. Sunstone shows aventurescence, a glitter caused by flat platelets of included mineral. Tiger's eye shows chatoyancy, a moving band of light from parallel fibrous inclusions, and star sapphire shows asterism from rutile needles meeting at angles.
How to check any of them before buying
- Tilt the stone. If the color moves as the stone turns under one light, you are looking at an angle-dependent effect such as diffraction, interference, or chatoyancy.
- Change the light source. If the body color shifts between daylight and warm indoor light, that is color change.
- Check the cut. Play-of-color, asterism, and adularescence need a dome above them, so a faceted stone is showing you something else.
- Ask what the effect looks like in each lighting condition, and ask for photographs taken in both.
Ask for photographs of any color-change or phenomenal stone in both daylight and warm indoor light, and judge the effect rather than the listing's description of it.
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