1. A Rock Is a Mix of Minerals, and a Gem Is a Mineral That Passes Three Tests
Most rocks are mixtures. A piece of granite, for example, is a patchwork of different mineral grains pressed together. A mineral is a natural solid with a particular chemical makeup and a regular pattern of atoms. So a rock is like a fruit salad, and the minerals are the pieces of fruit.
A gem is not a special kind of material. The U.S. Geological Survey says that "gem" and "gemstone" mean any mineral or organic material (pearl and petrified wood, for example) used for decoration or display because it has beauty, rarity and durability [1]. Of about 2,700 mineral species, only about 100 have all three [1]. Durability matters because a stone worn every day gets knocked and scratched, so a harder stone lasts longer. Quartz, for example, has a Mohs hardness of 7, which is enough for rings [6]. So gems are chosen from ordinary minerals. The rest of this article asks what moves a piece of an ordinary mineral over the line.
2. Ruby and Sapphire Are the Same Mineral With Different Trace Elements
Ruby and sapphire look nothing alike, yet the Gemological Institute of America (GIA) says both are varieties of one mineral, corundum [2][3]. Corundum is made only of aluminum and oxygen, and in its purest form it is colorless [2][3]. The color comes from tiny amounts of other elements tucked into the crystal. Chromium makes ruby red, and the more chromium, the stronger the red [2]. Iron and titanium together make blue sapphire, and only a few hundredths of a percent can be enough [3]. Sapphire can also be pink, yellow, green or other colors, and the name is used for any corundum that is not red enough to be called ruby [3].
Corundum that never becomes a gem
Plenty of corundum is not jewelry at all. Corundum is the standard mineral for hardness 9 on the Mohs scale, among the hardest minerals, which makes it a very good abrasive [4]. A rock called emery, rich in corundum and usually mixed with other minerals such as magnetite, was long the most common natural corundum abrasive, until manufactured abrasives such as silicon carbide mostly replaced it [4]. Same mineral, two very different jobs. What decides the job is color, clarity and crystal quality.
3. Amethyst Is Quartz Whose Iron Was Changed by Natural Radiation
Quartz works the same way. GIA calls amethyst the purple variety of the mineral quartz [5]. Geologist Hobart King explains that the purple is thought to come from a little iron in the quartz, changed by natural radiation from the surrounding rock [6]. So amethyst is not simply quartz with a purple dye. Its color seems to need two things: a bit of iron and a dose of natural radiation.
That color can be fragile. Some amethyst can fade after long exposure to strong light, and heating can turn it yellow or golden. Much of the quartz sold as "citrine" is actually heated amethyst [6]. If you ever see a purple stone in a sunny shop window, you now know why it may be wise to keep it out of strong light.
4. Big, Clear Crystals Usually Need Open Space to Grow
Color is only part of it. A gem also needs to be clear and big enough to cut. In ordinary rocks, mineral grains grow packed against each other and stay small. Many gem crystals instead grow in open pockets. The most important amethyst deposits are found in fractures and cavities of igneous rock, and in Brazil and Uruguay huge amounts formed in cavities in basalt lava flows. Small cavities, called geodes, are often cut open to show the crystals inside, and large ones can hold hundreds of pounds to several tons of amethyst [6]. In granite-like rocks called pegmatites, gem minerals such as tourmaline, beryl, topaz and quartz are also often reported in cavities where gas and fluid collected [7].
Why those pockets grow such large crystals is still debated. A common story says the rock simply cooled slowly. But a 2020 study in Nature Communications of quartz from pockets in a California pegmatite found chemical clues of surprisingly fast growth, and its authors say how quickly giant pegmatite crystals grow is still poorly known [7]. So the safe version is this: room to grow matters, and the full recipe is still being worked out by scientists.
5. Why Diamonds Need a Special Volcanic Eruption to Reach the Surface
Diamond is the odd one out. It is almost pure carbon, about 99.95 percent, and it forms only where it is extremely hot and the pressure is crushing, in a depth range of about 100 miles (roughly 160 kilometers) below the surface [8]. Graphite, the "lead" in a pencil, is also pure carbon, but its atoms are arranged differently, so it is soft enough to write with while diamond can only be scratched by another diamond [8]. That is another reminder that what a mineral is made of is only half the story. How the atoms are arranged matters too.
So how does something formed so deep get to a place where people can find it? Many diamonds are brought up in a rare kind of magma called kimberlite. In 2012 a team led by volcanologist Kelly Russell of the University of British Columbia reported a lab finding: when a carbonate-rich melt like kimberlite meets silica-rich rock, the two react and release carbon dioxide gas, and the experiment began bubbling within about 20 minutes [9]. The researchers proposed that this gas helps speed the magma, and any diamonds in it, toward the surface, and that this may help explain why diamonds arrive so abruptly [9]. The trip from the depths is described as taking hours to days [9]. The exact speed is still being studied, and different studies give different numbers, so treat any single figure with caution.
So a diamond needs the right chemistry, the right depth, and then a rare kind of eruption to carry it up. That is part of what makes it rare.
6. How to Look for the Same Differences Yourself
You do not need a lab to start. At a natural history museum or rock shop, find a gem crystal and a plain rock containing a related mineral, and compare how clear they are and whether you can see separate grains. If you have an adult to help, you can grow salt crystals by dissolving salt in warm water and letting it dry slowly in a shallow dish. Try a fast-drying dish and a slow-drying dish and compare the crystals. One question to take away: if a few atoms and some space can make a gem, what else around you is "ordinary" only because of where it grew?