Most Yoni Eggs are made of gemstones. These stones are also called gems, jewels, crystals, quartz, precious stones, or semi-precious stones. They are pieces of mineral crystal mostly found as rocks or in other rocks which, cut and polished, are used to make artifacts, such as decorative and ceremonial objects, jewelry, and objects for daily use, among others.
Certain rocks (such as, for instance, opal) and sometimes also other organic materials which are not minerals (think of pearl and amber) are also considered gemstones.
The majority of gemstones is rather hard, but you can also find soft gemstones. Typically, the more seldom a gemstone occurs in nature, the more value it has. Gemstones can come in many (grades of) colors and can have a wide variety of different characteristics, such as the level of transparency, the density, hardness, smoothness, glow, and so on.
Some gemstones are not suitable to be used inside the body, because they may break down while wearing them, releasing pieces of crystal into the vagina. Other crystals can be toxic for the body. To be really on the safe side, use well-known Yoni Egg crystals, such as Jade, Obsidian, or Rose Quartz.