Natural gems refer to gemstones that are formed in nature through natural geological processes, without human intervention. These gems are prized for their beauty, rarity, and durability. Some well-known examples of natural gems include:

1. Diamond: Formed deep within the Earth under high pressure and temperature conditions, diamonds are prized for their brilliance and hardness.

2. Emerald: A variety of the mineral beryl, emeralds are known for their vivid green color. They often contain inclusions which are considered part of their character.

3. Ruby: A red variety of the mineral corundum, rubies are highly valued for their rich color and rarity.

4. Sapphire: Another variety of corundum, sapphires come in various colors, with blue being the most famous. They are prized for their hardness and brilliance.

5. Opal: Known for its iridescent play-of-color, opals are formed from silica-rich solutions that fill voids or fissures in rock formations.

6. Topaz: Topaz occurs in a range of colors, but the most valued are the golden yellow to orange hues. It forms in igneous rocks and cavities within granitic pegmatites.

7. Amethyst: A purple variety of quartz, amethyst gets its color from irradiation, iron impurities, and the presence of trace elements. It is relatively abundant compared to some other gems.

8. Aquamarine: A blue variety of beryl, aquamarine is known for its light blue to blue-green color, reminiscent of the sea.

These are just a few examples, as there are many other types of natural gems, each with its own unique characteristics and formation processes.