49 Fun Facts About Rocks and Minerals

Which of these is NOT one of the three main groups of rocks?

Tectonic describes plate movement, not a rock type; the three groups cycle into one another over geological time in the rock cycle.

Igneous rocks form when what cools and hardens?

Magma or lava cooling and hardening forms igneous rock; slow cooling deep underground gives coarse-grained granite, while rapid cooling at the surface gives fine-grained rock.

Roughly what share of the Earth's crust, by volume, is igneous rock?

Igneous rock makes up about 65% of the Earth's crust by volume; layered rocks are only about 8 percent by volume despite covering most of the land surface.

Which rock, formed from compacted mud, makes up about 82 percent of the crust's layered rocks?

Two other layered rocks account for most of the rest.

Metamorphic rock forms when existing rock is transformed by heat and pressure without what?

Significant melting would make the result an igneous rock instead; metamorphism alters rock in the solid state at temperatures above roughly 150 to 200 degrees Celsius.

Silicate minerals make up roughly what proportion of the Earth's crust?

Silicate minerals, built around tetrahedra of one silicon atom bonded to four oxygen atoms, make up about 95% of the Earth's crust.

The Mohs hardness scale was devised in 1812 by a mineralogist from which country?

Friedrich Mohs, born in Gernrode in Germany, ranked ten minerals by which could scratch which; the scale is ordinal, so the steps between minerals are far from evenly spaced.

Which mineral sits at number 1, the softest, on the Mohs scale?

A fingernail rates about 2.5, so it can scratch talc and gypsum but not calcite.

Which mineral is number 9 on the Mohs scale, just below diamond?

Topaz is 8 and quartz 7; a steel knife blade and window glass both come in around 5.5.

Which soft mineral is heated to make plaster of Paris?

Gypsum, a hydrated calcium sulfate, is heated to drive off most of its water and make plaster of Paris; it rates 2 on the Mohs scale, soft enough to scratch with a fingernail.

Which grey metallic mineral is the principal ore of lead?

Galena, lead sulfide, is the principal ore of lead; its dense, cubic, metallic-grey crystals also served as the detector in early crystal radio sets.

Emerald is a green variety of which mineral?

Beryl, a beryllium aluminium silicate, becomes emerald when traces of chromium or vanadium colour it green; the same mineral in blue is aquamarine.

Diamond is a crystalline form of which element?

Carbon atoms bonded in a rigid tetrahedral lattice make diamond; it forms 150 to 250 km down in the mantle and reaches the surface in volcanic rocks called kimberlite.

The word 'diamond' comes from the Ancient Greek adamas, meaning what?

Adamas meant 'Unbreakable' or untameable, a nod to the stone's hardness; diamonds have been known in India for at least 3,000 years.

How many carats did the Cullinan diamond weigh when dug up in South Africa in 1905?

The rough Cullinan weighed 3,106 carats and was presented to King Edward VII; its largest cut stone, the 530-carat Great Star of Africa, sits in the Sovereign's Sceptre.

Which mineral defines hardness 7 on the Mohs scale and keeps time in most modern watches?

It is silicon dioxide, the second most common mineral in the crust after feldspar, and the first quartz clock was built in 1927.

Which mineral group splits into thin, flexible, transparent sheets once used as window panes?

Mica minerals such as muscovite cleave into thin, flexible, transparent sheets that once served as window panes and as the windows of stoves and lanterns.

Which volcanic rock has so many gas bubbles that it floats on water?

Pumice is frothy volcanic rock whose trapped gas bubbles make it less dense than water; pumice stones are also used to stone-wash jeans and to scrub skin.

Obsidian is a natural volcanic what?

Glass is what obsidian is: silica-rich lava that cooled too fast for crystals to grow, and its fractured edges can be many times sharper than a steel scalpel.

Marble is the metamorphosed form of which rock?

Limestone recrystallises under heat and pressure into marble; Michelangelo carved Carrara marble from Tuscany, and the Parthenon was built of Pentelic marble from near Athens.

The word 'marble' comes from a Greek word meaning what?

Shining stone is the sense of the Greek marmaros, from a verb meaning to sparkle; marble's subsurface scattering of light gives statues their lifelike, waxy glow.

Which black, polishable form of coal was carved into Victorian mourning jewellery?

Jet, a dense black fossilised wood related to lignite coal, takes a high polish; Whitby jet became mourning jewellery after Queen Victoria wore it following Prince Albert's death in 1861.

The White Cliffs of Dover are made of chalk, which formed from the remains of what?

Microscopic plankton called coccolithophores shed tiny calcite plates that settled on the sea floor over tens of millions of years, compacting into the chalk of the cliffs.

Which geological period takes its name from the Latin word for chalk?

Cretaceous comes from the Latin creta, chalk, after the thick chalk beds of that age across western Europe; the period ran from about 145 to 66 million years ago.

Halite, the mineral spread on icy roads, is the natural form of what?

Sodium chloride is the chemistry of halite, or rock salt; it forms cubic crystals when seawater evaporates, and the name comes from the Greek hals, salt.

Coal is classed as what kind of rock?

Sedimentary rock is coal's class: it forms from layers of plant matter buried and compressed in ancient swamps over millions of years, which is why seams often hold fossil leaves.

Which is the highest-quality rank of coal?

Temperature during formation matters more than pressure or time; in 1700 five-sixths of the world's coal came from Britain.

A hollow rock lined inside with crystals is called what?

A geode forms when crystals grow inward from the walls of a cavity; the Pulpí geode in Spain, found in 1999, is about 8 metres long and people can crawl inside it.

Which ore rock is the world's main source of aluminium?

Bauxite, named after Les Baux in southern France where it was first described in 1821, is refined into alumina and then smelted into aluminium metal.

Uluru, the giant monolith in central Australia, is made of what kind of rock?

Sandstone of a coarse, iron-stained kind called arkose makes up Uluru, which rises 348 metres above the plain and glows red at dawn and dusk; climbing was banned in 2019.

Which rock formation became the first US National Monument in 1906?

Devils Tower, a Wyoming column of phonolite porphyry, was proclaimed the first US National Monument by Theodore Roosevelt on 24 September 1906; Close Encounters of the Third Kind sent visitor numbers soaring in 1977.

The Rosetta Stone, key to decoding hieroglyphs, is a slab of which igneous rock?

Granodiorite, a grey granitic rock, is the slab's material; French soldiers found it at Rashid in 1799, the British took it in 1801, and it has sat in the British Museum since 1802.

The oldest known intact crustal rock on Earth, the Acasta Gneiss, is found in which country?

Canada is home to the Acasta Gneiss, about 4.03 billion years old, exposed on an island in the Acasta River in the Northwest Territories, roughly 300 km north of Yellowknife.

The oldest known mineral grains on Earth, zircons from Jack Hills, are about how old?

At about 4.4 billion years old, the Jack Hills zircons from Western Australia are only around 150 million years younger than the planet itself.

The Hoba meteorite, the largest known intact meteorite at about 60 tonnes, lies in which country?

Namibia, near Grootfontein, is where the Hoba iron-nickel meteorite was found in 1920; it has never been moved from where it fell, and about 86 percent of meteorites are stony chondrites instead.

Most meteorites that land on Earth come from where?

The asteroid belt between Mars and Jupiter supplies most meteorites, as fragments knocked loose by collisions drift into Earth-crossing orbits; a small minority are pieces blasted off the Moon or Mars.

According to the modern American list standardised in 1912, which gem is a birthstone for October?

Opal was October's stone on the list the American National Association of Jewelers adopted at Kansas City in 1912; tourmaline was added as an alternative in 1952.

Which month's birthstone is the ruby?

July is ruby's month on the American birthstone list, while sapphire, the other gem form of the same mineral, belongs to September.

Which is the correct order of sediment sizes from smallest to largest?

Clay, silt, sand, gravel is the order on the Wentworth scale: clay particles are under 4 micrometres across, sand runs from 0.0625 to 2 mm, and anything over 2 mm is gravel.

What term describes natural solids like obsidian or amber that lack a crystal structure?

Mineraloid is the term for such amorphous substances; true minerals are crystalline solids with atoms bonded in an orderly lattice, which obsidian and amber lack.

For roughly how long have humans and other hominids been using rock as tools?

2.5 million years or more: the Oldowan chopping tools from Gona in Ethiopia, flaked from river cobbles, date to about 2.6 million years ago.

Roughly how many official mineral species did the International Mineralogical Association recognise as of May 2025?

About 6,100 species were on the IMA list by 2025, with roughly 100 new minerals approved each year; most are named after a person or the place of discovery, and nearly all names end in '-ite'.

Which of these is on the IMA's official mineral list despite not being a crystalline solid?

Mercury, the only metal liquid at room temperature, was grandfathered onto the IMA list because native mercury was described long before the crystalline-solid rule; it freezes at about -39 °C.

Which mineral family makes up about 50 percent of the Earth's crust, the most abundant group of all?

Feldspars, a group of aluminium silicates including orthoclase and plagioclase, are the most abundant mineral family in the crust, found in granite and most other igneous rocks.

What is the colour of a mineral in powdered form called, usually tested on a porcelain plate?

Streak is the colour of the powder left when a mineral is rubbed on unglazed porcelain; hematite, whatever its surface colour, always leaves a reddish-brown streak.

Field geologists drop dilute hydrochloric acid on a rock to identify which mineral class, which fizzes?

Carbonates such as calcite react with the acid and fizz as CO2 bubbles escape; most other mineral classes, such as silicates and oxides, stay inert.

Which potassium-bearing counterpart of halite has a pronounced bitter taste?

Halite itself is table salt, and sulfide minerals give off a characteristic smell when fractured or powdered.

Which three minerals all share the formula Al2SiO5 but have different crystal structures?

Kyanite, andalusite and sillimanite are the three Al2SiO5 polymorphs; kyanite is triclinic while the other two are orthorhombic, and kyanite's hardness even varies with direction.

In which 1735 book did Carl Linnaeus divide the natural world into plant, animal and mineral kingdoms?

Systema Naturae, first published in 1735 as an eleven-page outline, sorted nature into the kingdoms Regnum Animale, Regnum Vegetabile and Regnum Lapideum; Agricola's De Natura Fossilium had classified minerals in 1546.