Spectral Micrometer
From Star Trek : Freedom's Wiki
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 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. (ENT: "These Are the Voyages...")