This rocks trivia quiz has 50 free questions with answers and covers geology from the ground up: the three rock types and the rock cycle, granite and basalt, pumice that floats, obsidian sharp enough for surgery, marble and slate and chalk, the Mohs hardness scale from talc to diamond, why ruby and sapphire are the same mineral, quartz in your watch, halite on the roads, geodes, coal and meteorites. It also takes in the famous rocks of the world, from the Giant's Causeway and Uluru to Devils Tower, the Rosetta Stone and the Cullinan diamond, and the oldest rocks and mineral grains ever found. It is written for school science classes, rockhounds, pub quiz teams and anyone who picked up a shiny pebble on holiday and wanted to know what it was. Easy questions cover the rock types every student learns; the hard ones ask about crustal percentages, kimberlite and the year Friedrich Mohs published his scale. Every answer has been checked against a reliable source and each question shows its citation once you have answered.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Which of these is NOT one of the three main groups of rocks?
Tectonic
Tectonic describes plate movement, not a rock type; the three groups cycle into one another over geological time in the rock cycle.
Q 02Igneous rocks form when what cools and hardens?
Magma or lava
Slow cooling underground gives granite; fast cooling at the surface gives glassy or fine-grained rocks.
Q 03Roughly what share of the Earth's crust, by volume, is igneous rock?
65%
Sedimentary rocks make up only about 8 percent of the crust by volume, though they cover most of the land surface.
Q 04The oceanic crust is about 99 percent made of which dark igneous rock?
Basalt
The continents, by contrast, are dominated by granite and similar rocks called granitoids.
Q 05Which rock, formed from compacted mud, makes up about 82 percent of the crust's layered rocks?
Shale
Two other layered rocks account for most of the rest.
Q 06Metamorphic rock forms when existing rock is transformed by heat and pressure without what?
Significant melting
It takes temperatures above about 150 to 200 degrees Celsius and pressures over 1,500 bars.
Q 07Silicate minerals make up roughly what proportion of the Earth's crust?
95%
They are built around silica tetrahedra and account for about a third of all known mineral species.
Q 08The Mohs hardness scale was devised in 1812 by a mineralogist from which country?
Germany
Friedrich Mohs ranked ten minerals by which could scratch which; the scale is ordinal, so diamond is actually about four times harder than corundum.
Q 09Which mineral sits at number 1, the softest, on the Mohs scale?
Talc
A fingernail rates about 2.5, so it can scratch talc and gypsum but not calcite.
Q 10Which mineral is number 9 on the Mohs scale, just below diamond?
Corundum
Topaz is 8 and quartz 7; a steel knife blade and window glass both come in around 5.5.
Q 11Ruby and sapphire are both gem varieties of which mineral?
Corundum
It is aluminium oxide; chromium makes it red and earns it the name ruby, while every other colour is called sapphire.
Q 12The gritty abrasive called emery, used on sandpaper and nail files, is an impure form of which mineral?
Corundum
Corundum axes dating to 2500 BC have been found in China.
Q 13Emerald is a green variety of which mineral?
Beryl
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?
Q 21Marble is the metamorphosed form of which rock?
Limestone
The Parthenon was built from Pentelic marble, the Taj Mahal from Makrana marble, and Michelangelo carved Carrara marble from Tuscany.
Q 22The word 'marble' comes from a Greek word meaning what?
Shining stone
Its subsurface scattering of light gives statues a lifelike, waxy glow; China and Italy lead world production.
Q 23Slate, used for roofs and old blackboards, is the metamorphosed form of which rock?
Shale
Its 'slaty cleavage' lets it split into flat sheets; by the 2010s Spain supplied about 90 percent of Europe's roofing slate.
Carbon
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.
Q 15The word 'diamond' comes from the Ancient Greek adamas, meaning what?
Unbreakable
Diamonds have been known in India for at least 3,000 years and also have the highest thermal conductivity of any natural material.
Q 16How many carats did the Cullinan diamond weigh when dug up in South Africa in 1905?
3,106
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.
Q 17Which mineral defines hardness 7 on the Mohs scale and keeps time in most modern watches?
Quartz
It is silicon dioxide, the second most common mineral in the crust after feldspar, and the first quartz clock was built in 1927.
Q 18Amethyst is the purple variety of which mineral?
Quartz
Citrine is the yellow version, rose quartz the pink and smoky quartz the grey-black.
Q 19Which volcanic rock has so many gas bubbles that it floats on water?
Pumice
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.
Q 20Obsidian is a natural volcanic what?
Glass
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.
Q 24The White Cliffs of Dover are made of chalk, which formed from the remains of what?
Microscopic plankton
The Cretaceous period is named after these deposits, from the Latin for chalk; modern blackboard chalk is often gypsum instead.
Q 25Which geological period takes its name from the Latin word for chalk?
Cretaceous
Chalk is typically 35 to 47 percent pore space and fizzes when acid is dropped on it.
Q 26Halite, the mineral spread on icy roads, is the natural form of what?
Sodium chloride
It forms cubic crystals when seawater evaporates, and its name comes from the Greek hals, salt.
Q 27Coal is classed as what kind of rock?
Sedimentary
It forms from plant matter buried in ancient swamps; China mines almost half the world's supply.
Q 28Which is the highest-quality rank of coal?
Anthracite
Temperature during formation matters more than pressure or time; in 1700 five-sixths of the world's coal came from Britain.
Q 29A hollow rock lined inside with crystals is called what?
A geode
The Pulpí geode in Spain, found in 1999, is a crystal cave about 26 feet long that people can crawl inside.
Q 30The Giant's Causeway in Northern Ireland is made of roughly 40,000 interlocking columns of which rock?
Basalt
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.