Zircon
Zircon has a high refractive index, which means that it can sometimes exhibit a lustre similar to diamond or sapphire. It has a hardness ranging between 6.5 to 7.5 on the Mohs Scale, making it fairly resistant to scratching and hence quite a versatile gemstone for jewellery.
Zircon is found in a few countries in south-east Asia (Cambodia, Vietnam, Thailand) and Africa (Madagascar, Mozambique, Nigeria). It also occurs in Sri Lanka, Brazil and Australia. It is mainly mined from secondary (alluvial) deposits.
This gemstone comes in a range of colours (blue, green, yellow, brown, red). It is often subjected to heat treatment which can improve, change or remove the colour. Sometimes, zircon will also be irradiated to change the colour.
Zircon is actually the oldest known mineral on Earth. Considering that the Earth formed 4.56 billion years ago, the oldest zircon gemstones represent the earliest record of the Earth's crust. Zircon was the first crystal to form in molten granite as it cooled to form rock. The ancient granitic rock has long eroded away; the only record left is zircon in the form of tiny grains. This means that zircon is the oldest substance on Earth; the oldest samples are even older than the moon, which formed about 4 billion years ago.
Scientists have confirmed that a tiny zircon gemstone discovered on a remote rock outcrop in Australia is the oldest known material of any kind formed on Earth.
The tiny fragment of zircon was found in 2001 on an outback sheep station in Western Australia by University of Wisconsin geoscience professor John Valley. At the time, he had been on a field trip to an area called Jack Hills, located north of Perth.
An international team of scientists, led by Valley, conducted a study on the gemstone, which revealed that the Earth’s crust first formed at least 4.4 billion years ago, 160 million years after the formation of the solar system.
According to Valley, the research – published in the journal Nature Geoscience – showed that the time when the planet was a fiery ball covered in magma ocean came earlier.
“This confirms our view of how the Earth cooled and became habitable,” Valley said. “This may also help us understand how other habitable planets would form.”
The study, which used a new technique called atom-probe tomography, builds on earlier research that relied on lead isotopes to date Australian zircons and identify them as the oldest bits of the Earth’s crust.
Valley said that the study “reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago” and possibly life not long after.
The zircon in question measures only about 200 by 400 microns, approximately twice the diameter of a human hair
http://www.jewellermagazine.com/Article.aspx?id=3918
Many people confuse zircon with the artificial (man-made) diamond simulant, cubic zirconia (CZ). Of course, the two are completely different. CZ, is a lab-created synthetic material, while zircon is a naturally occurring gemstone with a very different chemical composition. This unfortunate and incorrect association has probably impacted its appeal in the jewellery sector.