Emerald, ruby, opal, gems and jewels. These words have been in our vocabulary for thousands of years. However, defining what a gemstone is has proven to be a major challenge. Not for everyday people with common sense, but for those lexicographers who have a need to precisely define each work in our language. This definition covers the vast majority of the things we regard as gems.
Most gems are minerals but some, notably pearls and amber, are organics.
That means they were created by living organisms. The coating of a pearl is mineral which is confusing some folks. By definition though, a mineral must be created in the earth. Hence, pearls fall into a different category. Amber began life as tree sap.