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Jubilee agate: a royal piece of quartz
When it is viewed with light shining from behind, this polished slice of agate reveals an internal pattern with a likeness of HM Queen Elizabeth II.
The specimen is currently on display at the Museum in the Minerals gallery as part of the celebrations of the Platinum Jubilee of the Queen.
A royal slice of agate has gone on display at the Museum.
The piece of agate, a type of chalcedony which itself is a variety of quartz, has a pattern resulting from different impurities such as iron oxides creating different colours of the banding. Agates commonly occur as geodes which are rounded nodules formed of concentric layers, sometimes with a hollow centre. The rounded shape of the original agate can be seen by the naturally curved outline of the slice.
Imagine the surprise and delight when this slice was first cut and lifted into the light allowing it to shine through and reveal the pattern created by this banding and irregular void.
The agate was originally bought by gem dealer Robert Holt, when he was travelling around Brazil on a gem buying trip. Whilst probably not what he set out to purchase, Holt recognised the likeness of the agate's internal pattern to a profile of HM Queen Elizabeth II and so bought it and brought the mineral back to the UK.
In 1975 Holt then presented the specimen to the Geological Museum of the British Geological Survey. The agate was then transferred to the Museum in 1985 when the British Geological Survey's mineral and gemstone collections were merged into the Museum's.
Be sure to visit the Mineral gallery to see this specimen over the Jubilee celebrations.
The Jubilee agate is currently on display in the Museum's minerals gallery. ©The Trustees of the Natural History Museum, London
100 million years in the making
This particular agate was found in Lajeado, which is in the Rio Grande do Sul state of southern Brazil. The area stretching from Rio Grande do Sul through to nearby Uruguay is world-famous for its deposits of agates and amethyst geodes.
Over 100 million years ago, when dinosaurs were still roaming the planet during the Early Cretaceous, magma erupted and vast lava flows covered this area of land. As the lava cooled into basaltic rock, bubbles of gas trapped within the lava created spherical voids in the solid rock.
Over the following tens of millions of years, water rich in silica passed through these cavities, and as it did tiny amounts of silica were deposited on the cavity walls. As this happened over and over it created concentric layers on the walls and eventually formed the agate we see today. Despite scientific research the exact method of how this occurs is still unknown.
The story of the formation of this particular agate can be determined by examining its layers.
Agate's form from the outside inwards, so the first layers to form were the yellow and white translucent banding around the edges. These layers grew from the silica onto the walls of the cavity, following its rounded shape.
Then a change in the geological conditions occurred, causing a layer of quartz crystals to grow inwards towards the centre of the cavity. Conditions changed again, perhaps returning to those at the start, and so concentric layers were again deposited onto the quartz crystals, until there was only a small irregular cavity remaining in the centre. This is lined by a final thin layer of tiny quartz crystals.
A geological sample that has a resemblance to something else is sometimes called a mimetolith.
