The Mark X is one of the numbered gemmological stereo-zoom microscopes in the American Mark series, sitting between the Mark VII and the later Mark XV. Like its siblings, it is a binocular instrument designed specifically for the inspection of gemstones and mounted jewellery rather than for general microscopy, and it represents the working tool that defined the small trade laboratory for several decades.

Standard configuration on the Mark X includes a stereo-zoom binocular head with magnification in the range of approximately 10x to 60x, integrated darkfield illumination beneath an iris diaphragm, brightfield bottom illumination, and overhead reflected lighting. An articulating stone holder is fitted to allow rotation of the specimen through any axis, which is essential for examination of inclusions in transparent stones and of surface features on cut, mounted or cast pieces.

The instrument is recognised in the gem trade as the everyday workhorse for treatment recognition, identification of common synthetics by inclusion suite, and routine clarity grading. Used examples remain in service in many independent appraisal practices and continue to be repaired and refurbished by gemmological-equipment specialists. Buyers should verify the precise optical specification, accessory complement and condition of any individual unit before purchase rather than relying on the model designation alone.