A pair of scientists have just documented more than 5,000 known minerals on Earth, and explained why some of them are so incredibly rare.

The pink mineral cobaltomenite is found in just four places in the world: Argentina, Bolivia, Congo, and Utah, and researchers have come up with an explanation for why this and other gems tend to be so difficult to find, according to a Los Angeles Times report.

The study was published in American Mineralogist and represent the first system for categorizing rare minerals.

Some minerals, like cobaltomenite, can only form under specific pressures and temperatures. Still others require elements that are rarely found in the same place. Some are created a bit easier, but are often dissolved by the rain or degraded by the sun. Still others are very difficult to find because they are formed in areas that are extremely tough to reach, like deep below the ocean’s waves or at the edge of volcanoes.

And some of the gems have a combination of the above features. For example, fingerite has only ever been found near the summit of Izalco volcano in El Salvador, and it is also made of rare elements and dissolves in the rain.

Rare minerals are more than just a curiosity: they can help researchers better understand the geology of our planet, and how important life is in determining what our mix of minerals is.

The paper is titled “On the Nature and Significance of Rarity in Mineralogy.” It notes that many gems we think of as precious are actually quite common in comparison.

“Diamond, ruby, emerald, and other precious gems are found at numerous localities and are sold in commercial quantities, and thus are not rare in the sense used in this contribution,” the paper states. “Uses of the word ‘rare’ in the context of ‘rare earth elements’ or ‘rare metals’ are similarly misleading, as many thousands of tons of these commodities are produced annually.”