A mineral has a stricter definition than most people expect, and the tests geologists use in the field are simpler than most people expect too.
A mineral is not just anything dug out of the ground. It has to occur naturally, be solid, have a chemical composition that stays within narrow limits, and have its atoms arranged in a repeating lattice. That last requirement is what makes crystals possible, and it also quietly disqualifies several things sold in mineral shops — amber is fossil resin, obsidian is glass with no ordered structure at all, and pearl is made by an animal. Roughly four thousand minerals have been described, but a handful of silicates make up most of the crust, and the gemstones people recognise are usually colour varieties of a much duller parent species. Ruby and sapphire are the same mineral with different trace impurities; emerald and aquamarine are another such pair. The questions here cover the definitions, the field tests and the chemistry behind the colours.
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
Play it insteadAnswer: D. A naturally occurring solid with a definite composition and an ordered crystal structure
Why: Coal and amber fail the test because they come from once-living matter, and obsidian fails because its atoms are disordered like those in glass.
Answer: C. Oxygen
Why: Silicon dioxide is the commonest mineral in the continental crust, and the tetrahedron it forms is the building block of the whole silicate family.
Answer: B. Calcite
Why: The reaction frees carbon dioxide from the carbonate. Field geologists carry a small bottle of dilute hydrochloric acid for exactly this test.
Answer: A. Corundum
Why: Traces of chromium turn it red and iron with titanium turns it blue. The mineral is aluminium oxide, and it is second only to diamond for hardness.
Answer: D. Beryl
Why: Both are a beryllium aluminium silicate, and the difference is trace chromium or vanadium in one case and iron in the other.
Answer: C. Cleavage
Why: Mica peels into sheets because it splits in one direction only. A mineral that breaks along irregular curved surfaces instead is said to fracture.
Answer: B. The colour of the powder left when the mineral is rubbed on unglazed porcelain
Why: It is far more reliable than surface colour, which impurities can change. Hematite looks black or silvery but always leaves a rusty red line.
Answer: A. Produced by a living mollusc rather than formed in rock
Why: The animal lays down nacre, thin sheets of aragonite bound with protein, and light interfering between those sheets produces the lustre.
Answer: D. Slow cooling of magma deep underground
Why: Slower cooling gives atoms time to migrate into a growing lattice. Granite and basalt have similar chemistry but grains you can and cannot see.
Answer: C. Evaporation of seawater or of salt lakes
Why: Thick beds of it record ancient seas that dried out. Because it flows slowly under pressure, it can rise through overlying rock as a salt dome.
Staying warm, finding the way home, sleeping safely. Every species faces the same short list, and the answers are inventive.
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