The crust is the outermost layer of a planet. It surrounds the mantle. 25.7 percent of Earth's crust, by mass, is made up of silicon.
The crust of a planet is often subject to seismic disturbances. Seismic regulators were used to control this activity. (TNG: "Time's Arrow"; DS9: "Let He Who Is Without Sin...")
Terra 10 is a planet with a crust that was entirely crystalline. (TAS: "The Terratin Incident")
In 2365, Wesley Crusher and a team of geologists discovered that a planet had become unstable because unusually high levels of dilithium in the crust had formed a matrix which created a piezoelectric effect that was tearing the planet apart. This effect translated geothermal energy into mechanical force. (TNG: "Pen Pals")
In 2371, thirty percent of the crust of the Founders' homeworld was destroyed by weapons fire from starships above during the Battle of the Omarion Nebula. (DS9: "The Die is Cast")