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Marks of Distinction: The Facts Behind Inclusions

Dec 26, 2017 9:42 AM   By Joyce Kauf
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RAPAPORT... Inclusions are a natural diamond’s signature. Alethea Inns, director of gemology and education at the American Gem Society (AGS), explains the fascinating facts of this phenomenon.

What are inclusions?

In the world of gemology, inclusions are defined as internal visible features in either a rough crystal or faceted gemstone. Inclusions are important in gemology because they can tell trained gemologists whether the gemstone is natural, synthetic, or treated, or even where it came from.

Inclusions are often referred to as “flaws,” or “imperfections,” but in reality, they are part of the natural geological process and an essential and fascinating part of gemology. Inclusions make each diamond unique, as no two diamonds are identical, and are even used by gemological laboratories to verify a diamond’s identity. A diamond with natural inclusions is like a snowflake.

Inclusions can consist of other mineral crystals that look like what they are called — for example, a break in the crystal lattice is called a feather because it can resemble the texture of a feather. A pinpoint inclusion is just that — it looks like a pinpoint — while thin, elongated inclusions are called needles.

How do a diamond’s imperfections enhance it as something unique and authentic?

Inclusions are often used to determine the relative value of a diamond, based on their size, nature, number, position in the diamond, and relief — or visibility — but they are also so much more. Inclusions tell us the story of the diamond — how it formed, what it went through in the earth, and how it got to the surface of the earth. In fact, for geologists and researchers, the most prized diamonds are those specimens with large, visible inclusions. Some geologists spend their entire careers studying one type of diamond inclusion and its implications. This is because diamonds act as vessels that can capture minerals from the mantle of the earth and can give us information about the growth environment, and even the age of our planet. The oldest diamonds have been dated at up to 3.5 billion years old — which is not something you can say about a laboratory-grown diamond.

How are they formed? What conditions in the earth need to be present?

Conditions have to be just right not only to form diamonds, but to form the inclusions that occur within them. Diamonds are generally theorized to have formed at depths greater than 150 kilometers, at pressures of around 5 GPa (gigapascals) and at 1,000 degrees Celsius, and need a specific chemistry in order to grow. This is why they are geologically rare. Diamonds are carried to the earth’s surface by magma — molten and semi-molten rock found beneath the earth’s surface — younger than the host rocks in which they were formed. These two rock types in which diamonds grow in the mantle are eclogite and peridotite, and occur at different depths in the earth.

Diamonds are carried to the earth’s surface in one of only three rare types of magma. The most important type of magma is known as kimberlite, such as the Diavik mine in Canada. The Argyle mine in Australia, famous for its pink diamonds, is derived from a rarer type of magma, called lamproite, with an entirely different geochemistry. Pink diamonds from Argyle often have coesite inclusions, which are high-relief transparent crystals.

How do inclusions manifest themselves in diamonds?

Most diamond inclusions are classified as syngenetic, which means the inclusion forms at the same time as its host. Pyrope garnet is an example of a syngenetic inclusion you may find in a diamond, and is usually a dark-reddish included crystal.

Furthermore, diamonds are classified into different types based on the amount of nitrogen that aggregates in the crystal lattice. Some diamond types can be categorized by inclusion types, such as Ib diamonds with clusters of dark “Ib needles.” Large cape diamonds with N2 and N3 aggregates often have high-relief transparent garnet crystals and fine, transparent linear growth, known as “transparent graining.”

While there are still many unanswered questions about how exactly diamonds form, their inclusions are a window into the history of the earth, and their journey to the surface. They are an opportunity to carry a piece of that history with you, so you can admire diamonds from the inside out.

This article was first published in a special supplement produced in collaboration with the Diamond Producers Association.
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Tags: AGS, Alethea Inns, American Gem Society, inclusions, Joyce Kauf
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