A spectral micrometer was a device used to measure spectral bands of light which were diffused through many of the known crystals found within the Federation. These included 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 can be found in any jewelry shop within the Federation. (ENT: "These Are the Voyages...")