Hornfels jewellery

By Hugh Mitchell-Tapping

A very expensive piece of unique jewellery

Have you ever wondered what the basement rocks of the Beibuwan or Gulf of Tonkin (located off-shore north of Hainan Island in south-western People’s Republic of China) look like under a microscope?

Well, here they are!

Hornfels crystals 1Hornfels crystals 2

Hornfels crystals 3
Images: crossed-polarised light with ¼ lambda plate ×160 magnification.

They are beautiful contact-metamorphic rocks or “Hornfels”.

This rock type is usually found in contact aureoles bordering igneous intrusions. These granitic intrusions are accompanied by copious chemically-active liquids and gases, which cause hydrothermal contact metamorphism of the surrounding country rocks.

The age of these rocks, by potassium/argon age-dating method, was 91.7 ± 4 million years before present, or Cretaceous.

How much did it cost to obtain a sample?

In excess of US $5 million to drill to 3,459 metres (55 days) to obtain this basement core.

After the microscopic analysis, what did I do with the core?

Throw it away?

Oh, no! I wanted a souvenir of this very expensive unique rock.

So now, my wife wears a highly-expensive, polished, piece of hornfels jewellery as a necklace.

Hornfels jewellery 1Hornfels jewellery 2

However, at parties, it is always a beautiful topic of conversation of high interest to all.

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