Semiprecious describes a kind of stone that is not very rare or expensive, but still valuable and pretty.
When something is semiprecious, it is usually thought of as nice, but not as nice as something that is very rare and expensive.
Examples:
She wears a necklace with semiprecious stones.
The semiprecious ring is not as expensive as the diamond one.
I found a semiprecious rock in the garden.
He gave her a semiprecious bracelet, but she wanted a diamond one.
The semiprecious stones in the jewelry sparkled like the stars in the sky.
The semiprecious necklace was her favorite piece of jewelry.