
Are you ready for a rock rumble?
Over the summer we’ll be pitching a few of our favourite pebbles in a head-to-head battle to be crowned champion.
We’ll have sixteen amazing pebbles each going through a series of knockout rounds. All you have to do is vote on your favourite each week. Will it be the humble yet versatile granite? Or will the more glamourous schist take the win? Feel free to share the finest specimens, tag your friends and do some cheerleading for your most-loved rock. Maybe you would even like to nominate a future contender?
We’ll be posting our rock rumbles every Friday. There will be a polling option on the day that will last until the next week, when we’ll share the winners of the latest round.
The draw

The Rock Rumble draw.
Meet the pebbles
Browse the gallery below to find out more about our sixteen contenders!

Basalt – Basalts are dark, fine grained volcanic rocks associated with runny (low viscosity) lava flows with a high iron content and low silica content. As basalts form in a wide variety of different environments, from mid-ocean ridge eruptions thousands of metres underwater, to cooling quickly as a near surface intrusion, to part of a supervolcano flood lava, they have a variety of chemical and mineral contents. The Giant’s Causeway in County Antrim, Northern Ireland is comprised of basalt columns formed as the rock cooled into hexagonal structures.

Brick – Ceramic materials such as brick have been around humans for almost 10,000 years. Brick refers to a type of block made of clay, fired in a kiln often at temperatures of over 900oC. Brick is the only truly anthropogenic material on this list and its value as a construction material is virtually unparalleled. Brick is light, easy to produce, extremely cheap, requires little mortar in its use, is of uniform strength and can be morphed into any shape.

Chalk – Chalk is a sedimentary rock that is made of the mineral calcite with layers of flint nodules and clay. It is a form of limestone made of microscopic fossils known as coccoliths. The chalk you are probably most familiar with is that used for chalkboards or crayons. This is made of the softer mineral gypsum (2 on the Mohs scale of hardness). Chalk is made of the harder mineral calcite (3 on the Mohs scale). Chalk is usually white in colour, occasionally grey, yellow, buff or red. It is a soft material that breaks easily and will leave a powdery white dust on your hands. It is often found close to chalk cliffs.

Coral limestone – Biogenic limestones are white sedimentary rocks, known as carbonates, comprised almost entirely of calcite or aragonite (depending on the formation environment). Coral limestones are carbonates formed as corals living on reefs such as the Bahamas bank die. Alongside millions of sea creatures their gradual build-up of skeletons and mud compress together via diagenesis to form a carbonate platform. Modern corals are primarily of the Scleractinia order, whilst older corals of the Palaeozoic are of the tabulate and rugose orders. Scleractinia corals are aragonitic, whilst older rugose and tabulate corals are calcite based.

Gneiss – Gneiss refers to a branch of metamorphic rocks which have undergone significant changes under high pressures and temperatures causing them to develop their characteristic gneissose banding. This pattern of alternating white and black bands occurs as the primarily mafic dark minerals and the primarily felsic whiter minerals. These are forcefully separated by compressive forces deep underneath the surface of the earth. Because gneiss needs to reach pressures in excess of 10,000 times the surface pressure of the earth and be brought back to the surface without being melted, the rock is usually found in areas with a great degree of thrust faulting such as the Moine thrust in Scotland.

Granite – Granite is a generally light coloured, coarse grained igneous rock known for its speckled, crystalline appearance and for being one of the hardest natural rock types. Granite forms as magma cools deep beneath the Earth’s surface over long periods of time, allowing it to form a strong interlocking crystal structure. Generally, granites consist of feldspar, quartz, mica and a smattering of other minor minerals, and have a high silica content relative to basalts. In the UK, examples of large batholiths of granite can be found on Dartmoor and Bodmin Moor.

Hornfels – Hornfels is a metamorphic rock that forms due to the intense heat of an igneous intrusion changing rocks such as shales and clays, limestones and igneous rocks. It is known as a ‘ringing rock’ as it sounds like a bell when struck with a hammer. It can be used to make a ‘lithophone’ (a stone xylophone). The Till Family gave concerts utilising a lithophone in Europe and America in the 19th Century, the original ‘rock band’!
Hornfels is black or bluish- or greenish-grey in colour and has very fine grained crystals. It is often speckled with porphyroblasts of minerals such as andalusite, cordierite, garnet and sillimanite. Faint sedimentary layers from the original rock may remain. They are notoriously hard to break with a broken surface that has a partially concave surface similar to flint.

Ironstone –
Ironstone is made of a sedimentary rock such as sandstone or limestone that contains more than 15% iron in minerals such as hematite. It was a valuable source of iron during the industrial revolution. It is no longer used to make iron as other sources were discovered that have a much greater proportion of iron (typically greater than 50%) that are more economical to work.
Ironstone is dark red, rusty brown or yellow in colour. It will often be medium-grained i.e. have sand-sized particles. It also feels heavier than other pebbles of sandstone or limestone of a similar size. Ironstone often occurs as hard, rounded nodules. It may also contain fossils of shellfish or plants.

Leucogabbro – Gabbro forms a large part of the planet’s crust underneath the oceans known as ‘oceanic crust’. It can also occur as large intrusions of magma deep underground known as ‘lopoliths’ which can be revealed at the surface of the earth by erosion over millions of years.
Gabbro is a hard igneous rock formed from magma that cools slowly underground forming large crystals of augite, feldspar and the green mineral olivine. It occurs in a variety of colours most typically black and white, but also grey, dark greenish or bluish. It has large crystals (2 to 16mm) and often has much larger feldspar crystals (known as ‘phenocrysts’).

Limestone – Limestone refers to a broad range of sedimentary rocks comprised primarily of calcium carbonate. Once exposed to the atmosphere, weakly acidic rainwater dissolves the calcite, a process known as chemical weathering, to form karst landscapes and structures. Limestone makes for a great building material but is also vital to the steel industry as an additive to the steelmaking process. Limestone is also frequently associated with dolostone (primarily formed of the mineral dolomite, calcium magnesium carbonate). Varieties of limestone with UK examples include chalk (White Cliffs of Dover) and oolitic limestone (Cotswolds).

Phyllite – Phyllite is a type of metamorphosed mudstone, known for its unusual shine and wavy surface appearance. Sheets of mica have been realigned by pressure into horizontal, parallel planes giving the rock a flake-like quality. This gives the rock the strange property of being easily breakable along these planes, like peeling apart the pages of a book. Phyllites are associated with relatively low levels of metamorphism under temperatures around 300 to 450oC. Phyllites are largely comprised of micas such as muscovite, with the metamorphic process producing minor amounts of chlorite in the rock. They are found in the UK across the Grampian mountains and the Isle of Arran in western Scotland.

Rhomb porphyry – Porphyry refers to a texture found in igneous rocks, where coarse grained crystals are mixed in a background slush of tiny groundmass crystals, producing a two-tone texture. Larger crystals in porphyry are often referred to as phenocrysts. They are formed as a magma deep underground slowly cools down. The minerals with the highest melting point crystallise out first. Before the magma has time to completely solidify however, the process is interrupted in some way, either through eruption or sudden cooling, causing the remaining magma to crystalise quickly as a fine-grained groundmass. Rhomb porphyry is found in places such as the Oslo Rift Valley in Norway; the source of the rhomb porphyry pebbles on the east coast of the UK.

Sandstone – Sandstone refers to a wide variety of sedimentary rocks, but generally of those composed of quartz and feldspar grains between 0.0625 to 2mm, the size of sand grains you will find on a beach. Sandstone forms in a variety of environments, from desert sand dunes to river basins to beach-based marine deposits, making it a diverse and diagnostic rock type. Sandstone has several uses but primarily as a building material because it is resistant to weathering and often easy to work (think the Pyramids of Giza). The hidden value of sandstone however is its ability to hold liquids. Sandstone, when uncemented, is porous and permeable, allowing water and oil to pass through and be contained inside it, making it a prime host rock for aquifers and oil deposits.

Schist – Schist is a metamorphic rock that sits between phyllite and gneiss in terms of the temperatures and pressure the rock has undergone during regional metamorphism. This level of elevated pressure is often associated with mountain building events, where the tectonic plates atop the Earth’s mantle crush together. The rock is known for its flat, scale-like texture, formed as the mineral grains are preferentially layered by pressure. Schists are generally medium-grained, forming largely from mudstones and often contain a variety of metamorphic minerals such as garnet, chlorite and sillimanite.

Serptarian nodule – Septarian concretions are sedimentary and formed from the same material as the host rock. Minerals like calcite, quartz or iron oxide cement the rock together which makes it harder and able to survive intact after the host rock has been eroded away. They are the geological equivalent of a lucky dip. Carefully crack them open to reveal beautiful radiating crystals, a perfectly preserved ammonite or just solid rock.
Septarian nodules are black, brown, grey or yellow in colour. They often form spherical or irregular rounded shapes. They may have zig zag shaped radiating cracks on the surface infilled with a crystalline mineral (such as quartz or calcite).

Volcanic ash (Tuff) – Tuff is an igneous rock that is made of volcanic ash, a mixture of glass, mineral and rock fragments less than 2mm in size erupted from a volcano. It is cross over between an igneous and a sedimentary rock as volcanic ash is deposited like a sandstone. It may be termed a ‘volcaniclastic’ rock. Such small particles can have a big impact. The ash from the 2010 eruption of the Icelandic volcano Eyjafjallajökull in Iceland grounded many airlines and affected the travel of over 10 million people.
Tuff is mostly light to dark brown, grey to black in colour, occasionally with a pinkish, yellowish or greenish hue. It can resemble a sedimentary sandstone.
Guide to pebble spotting
Interested in finding out more about rocks? A great way to learn more is by pebble spotting! Pebbles are usually, but not always, formed from a naturally occurring rock that has been worn smooth by the action of water on beaches, lakes and rivers. They are also within reach of most people around the UK. For more information check out our pebble spotter’s guide which contains more information about how to get started and useful references for identifying your rocks.