What are agates?

Don’t be fooled by the dull outer appearance of agates (semi-precious gems). When they’re cut and polished, they reveal intricate structures and patterns.

Quartz, variety agate - dark grey and pale blue-grey fortification banded - one face polished - from Blue Hole, Usan, near Montrose, Angus
Quartz, variety agate – dark grey and pale blue-grey fortification banded – one face polished – from Blue Hole, Usan, near Montrose, Angus

The name comes from the ancient Greek name for the Dirillo River in Sicily – Achates, where large deposits were found. Although they can be found all over the world, Scotland has some of the most varied examples.

How did they form?

Hundreds of millions of years ago, volcanic eruptions caused lava to flow and cool over large areas of central and western ScotlandMost Scottish agates formed in the spaces created by gas bubbles trapped in the cooling lava.

The bubbles or cavities acted like moulds, which were then filled or partly filled with a mixture of different silica minerals. The cavities formed a variety of shapes; some were stretched and distorted by the partially molten lava, while others were squashed flat or partially joined together.

The material that forms agates is more durable than the surrounding rock, which can weather away after millions of years, leaving the agates behind.

Quartz, variety agate - amygdaloidal lava cut in two showing blue-grey agates - from rocky ravine halfway between Bonaly Tower and Bonally Reservoir, Pentland Hills, Midlothian
Quartz, variety agate – amygdaloidal lava cut in two showing blue-grey agates – from rocky ravine halfway between Bonaly Tower and Bonally Reservoir, Pentland Hills, Midlothian

Naming an agate

Varieties of agate are often given names reflecting their cut and appearance, such as flame, fortified and moss.

Flame agates have nothing to do with flames. Instead, they’re named for the shape of their banding.

Quartz, variety agate - many layered reddish-brown and pale grey "flame" banded with some common quartz infill - polished on one face - Agate book, F.61, p.28 - from Scurdie Ness, Montrose, Angus
Quartz, variety agate – many layered reddish-brown and pale grey “flame” banded with some common quartz infill – polished on one face – Agate book, F.61, p.28 – from Scurdie Ness, Montrose, Angus

Fortification agates, which are some of the most common varieties, resemble impenetrable fortified walls.

Quartz, variety agate - dark translucent grey with pale grey fortification banding, infilled with common quartz - from Burn Anne, Ayrshire
Quartz, variety agate – dark translucent grey with pale grey fortification banding, infilled with common quartz – from Burn Anne, Ayrshire

There are also moss agates, which have inclusions of dark green, black or red minerals. This gives them the appearance of moss or ferns.

Quartz, variety agate - purple, with reddish-brown and green mossing and a large common quartz infill - polished on one face - from Burn Anne, Ayrshire
Quartz, variety agate – purple, with reddish-brown and green mossing and a large common quartz infill – polished on one face – from Burn Anne, Ayrshire

Four agate facts

1) Professor Matthew Forster Heddle was Scotland’s most famous mineralogist. He collected thousands of agates, several of which are on display until early 2019. Many of the ones he collected were unusual or broken, which helped him investigate their formation.

2) Many agates have been collected from the Blue Hole, arguably Scotland’s most famous agate location, including this one (which looks like Bass Rock in East Lothian). However, the exact location is now unknown.

Quartz, variety agate - blue-grey and pale grey zoned and banded onyx with a smoky quartz infill - one face polished - from Blue Hole, Usan, Scurdie Ness, Montrose, Angus
Quartz, variety agate – blue-grey and pale grey zoned and banded onyx with a smoky quartz infill – one face polished – from Blue Hole, Usan, Scurdie Ness, Montrose, Angus

3) Lapidary, the cutting and polishing of gemstones and semi-precious stones, is one of the oldest human activities, dating back thousands of years. Worked agate has even been found in Neolithic sites in Scotland.

4) In addition to being beautiful, they’re hard and durable. Agates have been used for centuries to produce decorative items such as jewellery, as well as hardwearing items like mortars and pestles.

Gold ring brooch set with multi-coloured moss agates, with three gold thistles in the centre with heads of amethysts and a cairngorm
Gold ring brooch set with multi-coloured moss agates, with three gold thistles in the centre with heads of amethysts and a cairngorm

If you’d like to know more about agates, you can join a local lapidary club or visit Hidden Gems: Scotland’s Agates at the National Museum of Scotland in Edinburgh until 6 January 2019.


Header: Quartz, variety agate. A reddish brown carnelian with banding and a crystalline quartz centre. Towards the end of the formation process, quartz crystals may form instead of chalcedony. Blue Hole, Usan, Montrose, Angus.

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