Answer: Silicon

Starting in the 1990s, metrology institutes—organizations devoted to the scientific study of measurement—began collaborating on the Avogadro Project. This was an ambitious project intended to redefine the kilogram in the framework of a universal physical constant rather than the current method, which is to use the international prototype kilogram, a cylinder manufactured from a platinum–iridium alloy.

To that end, the Avogadro Project has created a perfect sphere of silicon with a mass of exactly 1 kilogram. This sphere, roughly the size of a softball, is so perfectly spherical that if you enlarged it to the size of the Earth, the variation in topography from the highest to lowest points on the silicon planet would be no more than 2.4 meters—as it stands, the sphere in its present size has a topographic variation of about 0.3 nanometers.

So why silicon? Thanks to the semiconductor industry, there was both ample existing research and actual industrial processes in place to allow for the creation of incredibly pure and imperfection-free silicon, leaving the scientists to the task of carefully cutting and polishing their silicon sample down to a perfect kilogram sphere.