A Spectral Micrometer is a device used to measure spectral bands of light that are diffused through many of the known crystals found within the Federation. These include such crystals as Diamonds, Dilithium and the many forms of Amethyst.
Within Starfleet, Spectral Micrometers are used to ensure the quality of Dilithium and other crystals used throughout starship operations. Outside of Starfleet, these instruments may be found in any jewelry shop within the Federation.
The word 'micrometer' is actually a misnomer as a micrometer literally means 'millionths of a meter.' Any good jewelry quality micrometer available in the Federation today can easily measure spectral features to the trillionth of a meter.