...is used for the gyroscopes in NASA's Gravity Probe B. Launched in April 2004, Gravity Probe B tests two effects predicted by Einstein's theory: the geodetic effect and the frame-dragging (see here for a brief intro).
In order for Gravity Probe B to measure these tiny effects, it must use a gyroscope that is nearly perfect—one that will not wobble or drift more than 10-12 degrees per hour while it is spinning.
"A nearly-perfect gyroscope must be nearly perfect in two ways: sphericity and homogeneity. Every point on its surface must be exactly the same distance from the center (a perfect sphere), and its structure must be identical from one side to the other [...]
After years of research and development, Gravity Probe B produced just such a gyroscope. It is a 1.5-inch sphere of fused quartz, polished and “lapped” to within a few atomic layers of perfect sphericity. A scan of its surface shows that only .01 microns separate the highest point from the lowest point. Transform the gyroscope into the size of the Earth and its highest mountains and deepest ocean trenches would be a mere eight feet from sea level!"