They're

  only

Adverb

  made of

  rock crystal

,  and

Coordinating conjunction

  there are

  millions

Noun

  of

Adposition

  rock crystals

  in

Adposition

  the

Determiner

  earth

Noun

.

They're only made of rock crystal, and there are millions of rock crystals in the earth.

They're only made of rock crystal, and there are millions of rock crystals in the earth.