The earth is unique in that we have a molten core, and we have periodic pole shifts, both are needed to form crystals. Crystals form when molecules are super heated and then cool. During pole shifts our plates get shuffled around opening up reservoirs of space which magma fills and begins it’s cooling process, as the magma cools over thousands of years under great pressure pushing the molecules closer and closer together crystals grow, we have hundreds of different varieties here from diamonds to salt, which are both examples of types of crystal.
Because we have such a perfect environment for growing crystal the earth has massive amounts of it inside, but the core itself is not a giant crystal. Crystals needs a cool environment to grow, thus the past volcanic passages protruding from the core are filled with amazing crystals some so big you could drive a car on it for miles perfectly flat, so the earth is unique in how much crystal there is inside.
The center of the earth is a near anti gravity environment there is not enough pressure to form crystals, and it is far to hot, crystals grow further out. In fact the greatest pressure and coolest environment is just under the outer crust, perfect for growing giant crystals and there are more crystals under us then vegetation on the surface.
A lot of people talk about the core being a giant crystal, even many scientists believe it cause they assume that gravity increases as you get closer to the core, but they don’t understand what gravity is to begin with, thus all their theories about it are not valid