Some of the more common semi-precious stones are alexandrite, agate, amethyst, aquamarine, garnet, lapis lazuli, moonstone, opal, pearl, peridot, rose quartz, spinel, tanzanite, tourmaline, turquoise, and zircon, and the list goes on. Every other gemstone that isn’t one of those four precious stones is considered to be semi-precious. 

This separation between precious and semi precious has no real scientific backing. For example, emeralds are a variety of beryl, as are aquamarines. An emerald is a precious stone, while an aquamarine is semi-precious. When this categorization came about, it was mainly due to the value and rarity of the 4 precious gemstones compared to the rest. Today, some semi precious gemstones can be worth much more than a precious stone. As an example, many natural pearls garner huge prices, often worth more than a low quality precious diamond, ruby, emerald, or sapphire. Spinels are another example. Additionally, many semi-precious stones can be more rare than some of the precious. Demantoid garnets or tsavorite garnets, and many other semi-precious gemstones that are hard to find, harder to mine, and produced in much lower quantities than precious gemstones.