Why Are Crystals Different Colors? The Real Causes

Why Are Crystals Different Colors?

Pure quartz is water-clear. Add a trace of iron and some natural radiation, and you get amethyst. Swap the iron for aluminum and you get smoky quartz. Same mineral, same crystal shape, completely different look.

Once you know the handful of reasons crystals have color, you can look at a stone and make a decent guess about what's going on inside it. It also helps with a practical question: is the color I'm looking at natural, or did someone add it?

The short answer: most crystals would be colorless if they were perfectly pure. Their color comes from tiny amounts of other elements trapped inside, from flaws in the crystal structure that trap electrons, or, in a few minerals, from an element that is part of the basic recipe. Light effects and treatments like heating, dyeing and coating account for most of the rest.

First, what "color" actually is

A crystal looks colored because it absorbs some wavelengths of light and lets the others through. What reaches your eye is whatever's left. A stone that soaks up red, orange and yellow light looks blue. One that absorbs everything except red looks red.

Everything below is about what inside a crystal does that absorbing.

Reason 1: a tiny impurity

This is the big one. A Caltech mineralogist named George Rossman has estimated that at least 40 percent of colored minerals get their color from just a handful of metal ions, according to a Rock & Gem magazine article on the subject. These ions, called transition metals, include iron, chromium, manganese, copper and titanium. They slip into the crystal in tiny amounts, sometimes a fraction of a percent, and change how it handles light.

Beryl is the best example, because one mineral becomes five different gemstones:

Corundum works the same way. Pure corundum is clear. A little chromium makes ruby, and other traces make every sapphire color except red. If you like that kind of detail, it's worth reading about how ruby and sapphire get their colors.

The same idea explains garnet, where changing which metals sit in the structure shifts the stone from deep red to bright green. Our garnet color guide walks through the varieties.

Reason 2: the same element can give different colors

Here's the part that surprises people. Chromium makes ruby red and emerald green. It's the same element in both stones.

The difference is the neighborhood. Each chromium atom sits among surrounding atoms, and their spacing and bonding change which wavelengths the chromium absorbs. In corundum it soaks up green and yellow, so you see red. In beryl the setting is slightly different, so it absorbs red and you see green. Geologists call this the crystal field, and it's why you can't predict a stone's color from its coloring element alone.

Reason 3: damage that makes color

Some colors don't come from an element at all. They come from flaws.

Inside a crystal, an electron can get knocked loose, usually by natural radiation in the surrounding rock over thousands or millions of years, and then get trapped at a spot where an atom is missing or out of place. That trapped electron absorbs light, and the crystal now has color. Mineralogists call these spots color centers.

• Smoky quartz gets its brown to gray color when natural radiation acts on aluminum impurities in the quartz.
• Amethyst gets its purple from traces of iron combined with this same kind of radiation effect.
• Fluorite owes its purple, green, blue and yellow shades to color centers, such as a fluorine ion that has shifted out of its normal position, along with impurity elements.
• Hackmanite is an extreme case, because its color centers form and fade within minutes of sunlight. If that sounds strange, we wrote about how hackmanite changes color.

One consequence is that this kind of color can be fragile. Heat or strong light can knock the trapped electrons back into place and the color fades. Fluorite, amethyst, topaz and kunzite are the usual examples, which is why collectors keep them out of direct sun.

Reason 4: the color is part of the recipe

A few minerals have a coloring element built right into their chemical formula. Mineralogists call these self-colored, or idiochromatic.

Peridot is green because iron is part of what peridot is. Malachite is green because copper is part of it. You don't find peridot in pink or malachite in blue, because the color comes with the mineral.

This is a good rule of thumb. If a mineral shows up in lots of colors, like quartz, fluorite, tourmaline or corundum, the color comes from something added. If it's nearly always the same color, like peridot or malachite, the color is part of the recipe.

Reason 5: light tricks, not pigment

Some of the most striking effects have nothing to do with absorbing light.

• Opal is built from tiny, evenly stacked silica spheres that split white light into a spectrum, the same principle as the rainbow on a CD. That's why the colors move when you tilt the stone.
• Labradorite flashes blue and gold because light bounces between ultra-thin internal layers.
• Aventurine glitters because of tiny flakes of another mineral, usually mica, trapped inside.

None of these colors is pigment. Move the stone and the color changes, which is the giveaway.

Why one crystal can have stripes

Crystals grow slowly, and the fluid they grow from doesn't stay the same. If its chemistry shifts mid-growth, the new layers pick up different trace elements and the color changes.

That's how a watermelon tourmaline ends up pink in the middle and green on the outside. It's also why ametrine is purple and yellow in the same crystal, and why fluorite often shows bands of color. The crystal is keeping a record of its own history.

Why two crystals of the same mineral can look different

Trace elements depend on what was dissolved in the fluids around the crystal, and that varies from one deposit to the next. So two crystals of the same mineral from two different mines can have different colors, or different strengths of the same color. Tourmaline is the clearest case, and our guide to tourmaline colors shows how wide the range gets.

What colors the crystals people buy most

Citrine is the honest surprise. Sources disagree about what colors it, and natural citrine is rare and usually pale. Most of the deep yellow-orange "citrine" sold today is amethyst that was heated. We cover the stone itself in our citrine guide.

Natural color, enhanced color, or added color?

This is where the science turns into a buying question. A crystal's color can come from the ground, from a lab, or from both. There are five common methods.

Two points are worth remembering, because they get lost in a lot of online advice.

Bright doesn't mean dyed. Fluorite, watermelon tourmaline and some tourmalines are naturally vivid. Hackmanite and kunzite are naturally colorful too. Some "rules" say any intense color must be dyed, and that's wrong.

Treated doesn't mean fake. Heated amethyst is still real quartz, and aura quartz is real quartz with a coating. The real problem is a seller who doesn't tell you. Treated stones are worth less than natural ones of the same type, and you should pay accordingly.

If a color looks too perfect, ask the seller directly whether the color is natural or treated. A good seller will answer in one sentence. If you're doing your own checking, start with the clues above, and keep our guide to telling real gemstones from fakes open as a second opinion.

One more thing: you can test for dye by rubbing a damp cotton swab on a hidden spot, but do it gently and only on a stone you don't mind marking.

Looking at color differently

Next time you pick up a crystal, run through a quick list. Is it a mineral that comes in many colors, or only one? Do the colors move when you tilt it? Are there stripes or zones? Is the color deepest near the tips, or does it sit in cracks? Those four questions tell you more than the color name does.

If you want to see color from different impurities side by side, browse our mineral specimens and crystals. The aquamarine crystals and tourmaline crystals collections are two of the easiest places to compare how one mineral changes color.

Frequently asked questions

Why do crystals come in different colors?

Most crystals would be colorless if perfectly pure. Their color comes from tiny amounts of other elements, from flaws that trap electrons (color centers), or from an element that's part of the mineral's recipe. Light effects and treatments cause the rest.

Why can one mineral be many different colors?

Because different trace elements and defects can sit inside the same crystal structure. Beryl is a good example, since iron, manganese and chromium turn the same mineral into aquamarine, morganite and emerald.

Why do some crystals fade in sunlight?

Color created by trapped electrons can be undone by strong light or heat. Fluorite, amethyst, topaz and kunzite are common examples, so they're best kept out of direct sun.

Are bright-colored crystals dyed?

Not necessarily. Some minerals, like fluorite and tourmaline, are naturally vivid. Dye is more likely if the color looks neon, collects in cracks, or rubs off on a swab.

Is citrine natural?

Natural citrine exists but is rare and usually pale. Most of the deeper yellow and orange citrine on the market is heat-treated amethyst, so ask the seller which you're getting.

What is aura quartz?

Natural quartz that has been coated with a thin layer of metal, such as titanium or gold, to give it a rainbow sheen. The quartz is real, but the color is added, so it should be sold as treated.