Fellows Auctioneers Ltd

What is Oregon Sunstone?

Oregon Sunstone is a calcium-rich plagioclase belonging to the extensive feldspar family. In other words, it is a native copper-bearing gem-quality labradorite, and its sparkling orange copper inclusions are the reason for its name.

History of Oregon Sunstone

Primarily found in the high desert regions of southeastern Oregon, especially in Lake County and Harney County, Oregon sunstones have an extensive geological history. They were initially collected from the ancient lava fields of Warner Valley. While the earliest written documentation dates back to the early 1900s, Native Americans had utilised these stones for centuries prior.

Formed over 13 million years ago during volcanic activity, Oregon sunstones originated from molten basaltic lava that cooled and solidified, trapping the native copper within, giving them their distinctive colour and shimmering effect.

Optical Effects

The distinctive "Schiller" effect, also known as aventurescence, is caused by these microscopic native copper inclusions that reflect light creating a glittering appearance. While copper is the predominant inclusion, other minerals like hematite also contribute to sunstone's unique optical qualities.

In the nanoscale, native copper inclusions influence the sunstone's colouration, producing a range of hues from yellow to orange, red, and even blue, depending on their concentration and orientation. Lower concentrations result in paler shades, such as light yellow; while higher concentrations yield rarer, more intense colours, including deep reds and greens.

Furthermore, Oregon sunstones can exhibit intriguing colour zoning, such as the "watermelon" effect.

A significant study published in the journal American Mineralogist in 2017 highlighted the discovery of protoenstatite—a mineral previously only theorised—in Oregon sunstones with watermelon colour zoning. This finding not only helps to explain the unique red-to-green pleochroism but also enhances the scientific value and appreciation of these gemstones.

Why is Oregon Sunstone more valuable?

Unlike sunstones from other localities, those found in Oregon tend to be smaller and higher quality, particularly suitable for faceting, carving, and setting into jewellery.

A noteworthy aspect of Oregon sunstones is the role copper platelets play in preventing fractures; they act like pebbles within concrete, halting fracture propagation and enhancing the stone's durability.

This feature, as well as hardness of 6 to 7 on the Mohs scale, their transparency, vibrant colour, pleochroism, and quality of aventurescence, make Oregon sunstones extremely desirable – especially the intense red or green specimens.

Since 1987, Oregon sunstones have been recognised as the official State of Oregon’s gem, hence the name, a true testament to their unique attributes and beauty.