Crust

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. 

Terra 10 is a planet with a crust that was entirely crystalline. 

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. 

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.