by Cathy Douglas
If you were to describe the minerals we sell at the store by chemistry alone, probably half of them would be made of the same thing: silicon dioxide (SiO2). In its purest form, silicon dioxide is clear quartz, also called rock crystal. But when this mineral contains trace elements of other minerals, or when the Earth’s inner forces have changed its form, we see a huge variety in appearance: everything from blue chalcedony to red jasper, and from amethyst to black onyx. The relationship between these forms can be very confusing. While these crystals’ properties are based on many factors besides composition, it can be interesting to know how they’re all related.
Quartz: When quartz is completely pure, it is either clear, white, or a combination of both. White, or “milky” quartz, is simply clear quartz that contains tiny gas bubbles. The more colorful varieties of quartz come from small particles of metallic elements that became trapped in the quartz as it hardened. Some are natural, some artificial:
- Smoky quartz contains traces of lithium and aluminum.
- Natural citrine also contains traces of lithium and aluminum, but in a different proportion. Less commonly it may come from amethyst naturally heated within the Earth.
- Amethyst contains traces of iron.
- Artificial citrine is usually made by heat treating amethyst, although it may also come from heating a relatively clear piece of smoky quartz.
- Rose quartz contains titanium, often as tiny bits of rutile (which is simply a needle-shaped formation of titanium dioxide).
- Oro verde quartz is a greenish-gold quartz formed through irradiation. Often when you see very dark “smoky quartz,” it’s also an irradiated stone.
- Aventurine is a kind of sparkly quartz formed by inclusions of mica. Most common is green aventurine, which contains fuchsite mica. Aventurines with a more reddish hue contain bits of hematite.
- Lithium quartz contains (obviously) lithium, often as inclusions of fuchsite mica.
- Blue tiger’s eye contains crocidolite (blue asbestos) fibers. When the iron in these fibers oxidizes, the stone bearing it becomes red or golden tiger’s eye.
Chalcedony: Color variation is only the beginning. Changes within the earth give us many more beautiful crystals, with an even greater variety of appearances. Pure quartz is clear because its crystal structure all lines up in the same direction as it forms. But of course this process is often disturbed, and the little crystals take random directions as they form. You’d only even be able to see them as “crystals” by looking through a microscope. (In crystal-nerd-speak, we say it’s “cryptocrystalline.”) This is chalcedony (pronounced cal-SED-a-nee), which has variations of its own:
- Pure chalcedony is white to gray to bluish in appearance.
- Chrysoprase is chalcedony that is green because of the presence of nickel.
- Carnelian is chalcedony which iron oxide gives an orange to red color.
- Bloodstone is a dark chalcedony containing iron silicates, plus splotches of red jasper.
- Black onyx is actually a very dark form of chalcedony. Sometimes it is made artificially by boiling chalcedony and treating it with acid.
Jasper: Jasper is chalcedony that has gone through even more changes within the Earth; the little mini-crystals have been roughed up and combined with other minerals. There are more varieties of jasper than any of us can count, but here are a few:
- Red jasper is chalcedony containing hematite.
- Yellow jasper combines chalcedony with clay.
- Bumblebee jasper contains yellow stripes of sulfur.
- There are many, many other jaspers of mottled appearance, with fanciful names such as Picasso jasper and dalmatian jasper. They’re formed when bits and pieces of various minerals get fused into the forming jasper.
Agate and Onyx: Banded chalcedonies go by two different names: When the layering is more or less parallel, the resulting stone is called onyx; when the banding is more circular or irregular, it’s called agate.
- Dendritic (branching) agates, such as moss agate, contain little branch-like threads of manganese oxide, iron oxide, slivers of other minerals, or sometimes just plain dirt.
- Orbicular jaspers, such as ocean jasper, are actually agates containing multicolored orbs which may include rhyolite, biotite, feldspar, and many other minerals.
- Onyx comes in flat black and white layers, so that a cross-section will appear to be striped.
- Sard is like carnelian, except that the color tends more toward brown. Its banded form is called sardonyx.
- You’ll often find brightly colored stones labeled as “onyx” or “agate” which are actually dyed.
Opal: Another thing that can happen as geology churns microcrystals is that they can get mixed up with water. If the crystal forms in an enclosed space where the water can’t evaporate, the crystal-plus-water gel hardens to form opal. This stone may contain other impurities as well, which is why opal comes in a rainbow of colors.
It’s hard to believe all this variety basically just comes from quartz! But since the majority of the Earth’s crust is made up of silicon-based minerals, maybe it’s not too surprising.