Precious stones are hard gemstones that are created naturally and need to be mined for. The four known precious stones are diamonds, rubies, sapphires, and emeralds. They are categorized as precious due to their aesthetics and economics, not because of their mineral content or chemical properties. These stones have characteristics including luster, hardness, fracture, cleavage, and dispersion. Precious stones have desired aesthetics that are often sought after and replicated on marble, porcelain, and ceramic tiles.
There is no real differentiation between the stones that are classified as precious and semi-precious. This categorization was determined in the 1800s based off of the revenue generated from the stones. All gemstones have grades or qualities that make them desirable, beautiful, and rare.