50 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?

Slow cooling underground gives granite; fast cooling at the surface gives glassy or fine-grained rocks.

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

Sedimentary rocks make up only about 8 percent of the crust by volume, though they cover most of the land surface.

The oceanic crust is about 99 percent made of which dark igneous rock?

The continents, by contrast, are dominated by granite and similar rocks called granitoids.

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?

It takes temperatures above about 150 to 200 degrees Celsius and pressures over 1,500 bars.

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

They are built around silica tetrahedra and account for about a third of all known mineral species.

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

Friedrich Mohs ranked ten minerals by which could scratch which; the scale is ordinal, so diamond is actually about four times harder than corundum.

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.

Ruby and sapphire are both gem varieties of which mineral?

It is aluminium oxide; chromium makes it red and earns it the name ruby, while every other colour is called sapphire.

The gritty abrasive called emery, used on sandpaper and nail files, is an impure form of which mineral?

Corundum axes dating to 2500 BC have been found in China.

Emerald is a green variety of which mineral?

Chromium or vanadium supplies the colour, Colombia is the historic leading producer, and the mossy inclusions inside are called the 'jardin'.

Diamond is a crystalline form of which element?

It forms 150 to 250 km down in the mantle and rides to the surface in volcanic rocks called kimberlite; the diamonds are usually far older than the rock that carries them.

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

Diamonds have been known in India for at least 3,000 years and also have the highest thermal conductivity of any natural material.

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

It was presented to King Edward VII, and its biggest 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.

Amethyst is the purple variety of which mineral?

Citrine is the yellow version, rose quartz the pink and smoky quartz the grey-black.

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

After Krakatoa erupted in 1883, rafts of it drifted around the Indian Ocean for up to 20 years; it also gives jeans their stone-wash.

Obsidian is a natural volcanic what?

It forms when silica-rich lava cools too fast for crystals to grow, and its edges can be many times sharper than a steel scalpel.

Marble is the metamorphosed form of which rock?

The Parthenon was built from Pentelic marble, the Taj Mahal from Makrana marble, and Michelangelo carved Carrara marble from Tuscany.

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

Its subsurface scattering of light gives statues a lifelike, waxy glow; China and Italy lead world production.

Slate, used for roofs and old blackboards, is the metamorphosed form of which rock?

Its 'slaty cleavage' lets it split into flat sheets; by the 2010s Spain supplied about 90 percent of Europe's roofing slate.

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

The Cretaceous period is named after these deposits, from the Latin for chalk; modern blackboard chalk is often gypsum instead.

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

Chalk is typically 35 to 47 percent pore space and fizzes when acid is dropped on it.

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

It forms cubic crystals when seawater evaporates, and its name comes from the Greek hals, salt.

Coal is classed as what kind of rock?

It forms from plant matter buried in ancient swamps; China mines almost half the world's supply.

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?

The Pulpí geode in Spain, found in 1999, is a crystal cave about 26 feet long that people can crawl inside.

The Giant's Causeway in Northern Ireland is made of roughly 40,000 interlocking columns of which rock?

Most columns are hexagonal, they formed 50 to 60 million years ago, and legend says the giant Finn McCool built them as a bridge to Scotland.

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

It rises 348 metres above the plain, glows red at dawn and dusk, and climbing was banned in 2019 out of respect for the Anangu.

Which Wyoming rock formation of hexagonal igneous columns became the first US National Monument in 1906?

Theodore Roosevelt signed it into being, and 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?

French soldiers found it in 1799, the British took it in 1801, and it has been in the British Museum since 1802.

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

It is about 4.03 billion years old and lies 300 km north of Yellowknife; older zircon grains from Jack Hills in Australia reach 4.4 billion years.

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

They are found in Western Australia and are only about 150 million years younger than the planet itself.

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

It is an iron meteorite; about 86 percent of meteorites are stony chondrites instead.

Most meteorites that land on Earth come from where?

About 70 percent seem to trace back to the break-up of just three asteroids.

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

Tourmaline shares October; the National Association of Jewelers set the list at a meeting in Kansas City in August 1912.

Which month's birthstone is the ruby?

September gets the sapphire, so the two forms of corundum sit two months apart.

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

Sandstone forms from the sand grade and shale from the clay and silt.

Volcanic glass like obsidian, lacking a crystal structure, is called not a mineral but a what?

True minerals are crystalline solids with atoms bonded in an orderly lattice.

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

Egyptians were quarrying soft sedimentary rock for buildings by about 4000 BC.

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

Most are named after a person, followed by the place of discovery, and nearly all names end in '-ite'.

Which liquid metal is still classed as a mineral by the IMA because it was listed before the current rules?

It only crystallises below -39 °C; water and carbon dioxide, by contrast, are not minerals.

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

Quartz is the single most abundant mineral species at about 12 percent of the crust.

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

It may differ from the mineral's body colour, which is often non-diagnostic.

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

The acid reacts with the carbonate group and gives off carbon dioxide gas.

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 is triclinic while the other two are orthorhombic; kyanite's hardness even varies with direction.

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

Georgius Agricola had offered an earlier mineral classification in De Natura Fossilium in 1546.