Calcium carbonate is not a complicated chemical formula. CaCO3. Three elements. And yet calcite has more described crystal forms than almost any other mineral on Earth, with catalogues running past 800 distinct habits.
One mineral, hundreds of outlines
A mineral species is defined by its chemistry and its internal crystal structure, not by its outward shape. The shape, what mineralogists call the habit, is decided by growth conditions: temperature, the chemistry of the fluid, how fast the crystal grew, and what impurities were present while it grew. Calcite is unusually sensitive to all four, which is why it produces such a spectacular range.
Three habits show up often enough to have earned nicknames.
Dogtooth
The scalenohedron. Steep, sharply pointed crystals that genuinely do resemble a canine tooth, often growing in clustered sprays off a matrix. Twelve faces, each a scalene triangle. Dogtooth calcite tends to form where growth was relatively fast and the fluid was rich, which is why it lines so many limestone cavities and Midwest quarry vugs.
Our dogtooth calcite from Ohio is the textbook version of this form, and it comes from the same broad Midwestern carbonate geology that produces so much of the classic American calcite.
Nailhead
A flat-topped rhombohedron capping a short prism, giving the crystal the profile of a nail driven partway into the rock. Nailhead spar generally indicates different growth chemistry to dogtooth, often slightly warmer or more magnesium-influenced fluids. The interesting part is that the two can occur in the same cavity, sometimes even on the same crystal, with a dogtooth core overgrown by a nailhead termination, recording a change in the water that was flowing through.
Rhomb
The blocky, leaning-cube shape that most people picture when they hear calcite. Iceland spar, the optically clear material famous for splitting an image in two, is usually a rhomb. That double-image effect has its own explanation in the post on birefringence.
The shape you get whether you want it or not
Here is the detail that ties it together. Whatever the external habit, calcite has perfect cleavage in three directions, and those cleavage planes meet at the same angles as a rhombohedron. Break a dogtooth point and it does not shatter into random chips, it separates into little rhombs. Break a nailhead crystal, same result. The internal structure is identical in all three; only the outside differs.
That is the cleanest possible demonstration of why cleavage and habit are different things, and it is a good reason to handle calcite carefully. At Mohs 3 with perfect cleavage, it is one of the more fragile things in a collection.
The golden ones
Calcite colour comes from trace elements and inclusions rather than from calcium carbonate itself, which is colourless when pure. The warm honey-gold Midwestern material gets its colour from trace iron. We carry it as both a single golden calcite point and a three-piece lot from Weston, Wood County, and the quarry behind that material has its own history, covered in the piece on the Weston quarry and in the older post on Ohio honey-gold calcite.
Why it matters when you are buying
Habit is one of the few things about a specimen you can assess honestly from a photograph. Colour photographs badly, size is deceptive, but a crisp dogtooth termination is a crisp dogtooth termination. When you learn to read habit, you stop buying calcite and start buying a particular calcite, which is a more interesting way to collect.
