Pick up a pebble, a crystal and a shiny bead from the same jar and you may wonder which is which. Scientists have simple answers: three words (rock, mineral, gemstone) and one famous ten-step ruler, the Mohs scale. Here is a plain guide for kids and the adults who help them, with sources.
The short version
- A mineral is a naturally formed solid with a crystal structure and a set chemical make-up. A rock is made of one or more minerals. A gemstone is usually a mineral crystal that has been cut or polished for jewellery.
- The Mohs scale ranks ten reference minerals from talc (1) to diamond (10) by what can scratch what. It is a ranking, not an even ruler.
- Colour alone is a poor clue. Geologists also look at lustre, streak, hardness and how a stone breaks.
Rocks and minerals for kids: what is the difference?
The U.S. Geological Survey (USGS) gives short definitions. A mineral is “a naturally occurring inorganic element or compound having an orderly internal structure.” A rock is “an aggregate of one or more minerals,” and common rocks include granite, basalt, limestone and sandstone.
The Smithsonian National Museum of Natural History uses a salad. If a rock is a salad, each mineral is an ingredient: you can separate them, and a carrot stays a carrot however small you chop it. Granite, it says, is made up of the minerals quartz, feldspar and other minor minerals.
Wikipedia’s article on rocks groups them into three families: igneous, sedimentary and metamorphic.
What makes a stone a gemstone?
A gemstone is a piece of mineral crystal that is cut or polished to make jewellery or other adornments, according to Wikipedia. The Smithsonian adds that mineral crystals valued for colour, clarity, beauty, durability and rarity may be suitable for fashioning into gems, and that a gem has facets, the sides cut and polished by a gem cutter.
The categories overlap. Wikipedia notes that certain rocks, such as lapis lazuli, opal and obsidian, and some organic materials that are not minerals, such as amber, jet and pearl, are also often considered gemstones. So a gemstone is a use and a quality, not a separate kind of material.
What is the Mohs scale?
Britannica defines Mohs hardness as a rough measure of the resistance of a smooth surface to scratching, expressed on a scale devised in 1812 by the German mineralogist Friedrich Mohs. Wikipedia adds that he introduced it in a book whose title translates as Attempt at an Elementary Method for the Natural-Historical Determination and Recognition of Fossils.
The idea is older than Mohs. Wikipedia says comparing hardness by seeing which mineral scratches another was mentioned by Theophrastus around 300 BC and by Pliny the Elder around AD 77. Mohs gave it numbers: his biography on Wikipedia says diamond was given 10, and softer minerals such as talc were given 1. Gypsum got 2 because it will scratch talc.
- Talc1
- Gypsum2
- Calcite3
- Fluorite4
- Apatite5
- Orthoclase (feldspar)6
- Quartz7
- Topaz8
- Corundum9
- Diamond10
Source: Britannica, “Mohs hardness”, and Wikipedia, “Mohs scale”, which list the same ten minerals and numbers.
To use it, you look for the hardest material your stone can scratch, or the softest that can scratch it. Britannica’s example: a mineral scratched by orthoclase but not by apatite has a hardness between 5 and 6.
The scale is a ranking, not a ruler
Do not read the bars above as equal steps. Wikipedia calls the Mohs scale an ordinal scale and gives this example: corundum (9) is twice as hard as topaz (8), but diamond (10) is about four times as hard as corundum, for absolute hardness. Britannica makes the same point: going from calcite to fluorite (3 to 4) is an increase of about 25 percent, while corundum to diamond (9 to 10) is more than 300 percent.
- 1 Talc1
- 2 Gypsum2
- 3 Calcite14
- 4 Fluorite21
- 5 Apatite48
- 6 Orthoclase72
- 7 Quartz100
- 8 Topaz200
- 9 Corundum400
- 10 Diamond1500
Source: Wikipedia, “Mohs scale”, table of absolute hardness measured by a sclerometer.
Britannica also says the Mohs test greatly helps with identifying minerals in the field but is not suitable for accurately gauging the hardness of industrial materials such as steel or ceramics.
How to identify gemstones and other stones: four clues
The Kansas Geological Survey, part of the University of Kansas, lists the common clues:
- Colour. It is the easiest to describe, and for some pure minerals it helps: gold is always yellow, turquoise always light blue. But impurities often change the colour of minerals.
- Lustre. This is the character of the light reflected by the mineral: metallic, glassy, earthy, pearly, silky and so on.
- Streak. Rub the mineral on unglazed porcelain and it leaves a streak of fine powder. The streak may differ in colour from the mineral itself, and the survey calls it an excellent check. Hematite always leaves a red-brown streak.
- Hardness. Try to scratch the stone with common objects: a fingernail has a hardness of 2.5, a pocketknife blade 5.5, and glass is slightly less than 6. Minerals with a hardness of 6 or more will easily scratch glass.
The same survey also lists crystal shape and cleavage, the tendency of some minerals to break along certain planes. One honest warning from Britannica: make sure a scratch is a real scratch, not a chalk mark that rubs off. And since a scratch test marks the softer stone, do it on a spare piece, not on one you love.
Why a gemstone collection is a good way to practise
These clues are easier to understand with real stones in your hand. Smithsonian geologists have identified more than 6,000 different minerals so far, so there is always another one to look up.
What we would pick
If you want stones to compare, our Gemstone Advent Calendar has 24 numbered doors with a tumbled gemstone or mineral behind each one. It comes in two collections, A and B, so siblings do not get the same stones. The recommended age is 6+; it has small parts, is not for children under 3 and is not for chewing. Natural minerals vary in colour and shape, so yours will look a little different from the photos. We do not list which stones are inside, and we do not state what any single stone is worth.
For a different kind of crystal shelf, our Crystal Solar System is a wooden box with colourful crystal planet stones, each with its own colour and pattern. It has small stones, so keep it away from children under 3.
Sources
- What is the difference between a rock and a mineral?, U.S. Geological Survey: the definitions of mineral and rock and the examples of common rocks.
- Gems and Minerals – Beauties and Building Blocks, Smithsonian National Museum of Natural History: the salad comparison, granite’s minerals, what makes a gem, facets, and more than 6,000 minerals identified.
- Mohs hardness, Britannica: the 1812 scale by Friedrich Mohs, the ten minerals and numbers, the orthoclase and apatite example, non-linearity (25 percent and more than 300 percent), field use and the scratch warning.
- Mohs scale, Wikipedia: the ordinal scale, the 1812 book title, Theophrastus and Pliny, the reference table and absolute hardness.
- Friedrich Mohs, Wikipedia: diamond given 10, talc 1, gypsum 2 because it scratches talc.
- Gemstone, Wikipedia: the definition, rocks and organic materials used as gems.
- Rock (geology), Wikipedia: igneous, sedimentary and metamorphic rocks.
- Kansas Rocks and Minerals: Minerals, Kansas Geological Survey, University of Kansas: colour, lustre, streak, hardness, everyday test objects and cleavage.
