Gemstones

Gems
There is no universal definition of gemstone. Most precious stones are minerals (e.g. diamond), more rarely they are mineral aggregates (e.g. lapis lazuli) or rocks (e.g. onyx marble). There are also some gemstones that are formed organically like amber, while others are synthetically formed.

Their outstanding beauty is a common feature. With some stones, it is the extraordinary color, the play of colors or the luster that makes the gemstones stand out from other stones. Sometimes it's the hardness or an interesting inclusion. The rare occurrence of the stone also plays a further role in its classification as a gemstone.

"semi-precious stone"
In the past, minerals that are notable for their beauty but are much more common in nature and are not as hard were called semi-precious stones. They were considered less valuable than precious stones. Today the term "semi-precious stone" is obsolete and is no longer used in mineralogy/gemmology, as it indicates a certain inferiority, which in fact does not exist.