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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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