The wavelength of visible light that is reflected from an object into the eye. The hue, i.e. “red”, “green”, etc. is determined by the wavelength of light. White light is the mixture of all colors.
Which wavelengths are reflected is determined by the properties of the object.
In gemstones, the color of the reflected light typically comes from the composition of metal elements within the stone or from variations in the crystal lattice structure of the stone itself.