But perhaps silicon?

The kilogram is the only one of the SI’s seven base units still defined in terms of a material artefact - a 130-year-old platinum-iridium cylinder maintained at the International Bureau of Weights and Measures in France. The US National Institute of Standards and Technology (NIST, nist 1) is working on a proposal that defines the kilogram in terms of the Planck constant h which is expressed in units containing the kilogram. Work at NIST such as the watt balance experiment (nist 2) and determinations of the mass of one mole of silicon atoms offer new ways of determining an accurate value of h, thereby contributing to a more reliable definition of the kilogram.