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Growing crystals are extremely sensitive to even minor changes in their environment. A misplaced atom—or a slight change in temperature, pressure, composition, or impurities—can produce strange and wonderful crystals.




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Overgrowths: Crystals Atop Crystals
Sometimes crystals of one mineral grow on top of crystals of a different mineral, producing an overgrowth. In most cases, you can still see the shapes of the crystals that were overgrown, even though they may have been completely covered or dissolved.

 

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Inclusions: Crystals within Crystals
Look closely for smaller pyrite crystals—or inclusions—inside the quartz crystal. How did they get there? In most cases, the inclusions formed first. The larger crystals grew around them later. Inclusions can also grow within a cooling crystal when some of its atoms separate to form a new mineral. Scientists analyze inclusions, most of which are microscopic, for clues to a crystal's growth history. Their presence also helps distinguish natural from synthetic gemstones.

 

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Pseudomorphs: Shapes That Deceive
A change in chemistry of a mineral's environment can trigger a chemical reaction, replacing that mineral with another, atom by atom. If the new mineral retains the crystal shape of the original, the specimen is called a pseudomorph—meaning "false form." Did you notice that the blue azurite crystal is slowly turning green? Azurite can react with water in the air or chemicals to become green malachite. After the transformation is complete, the crystal will be that of malachite, but in the crystal form of azurite.

 

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Phantoms: Ghosts of Crystals Past
Look for phantoms—ghostly outlines of the crystals' former shapes—inside the quartz crystal. Tiny bubbles or mineral grains coated earlier crystal faces that stopped growing. When they started growing again, the coating was trapped.

 

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Scepters
Two periods of growth produced these quartz crystals called scepters. First, the slender pedestals grew. Later, top-heavy caps formed, making the crystals look like royal staffs. No one knows exactly what conditions cause scepters to form.

 

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Twins
Crystal twins are groups of two or more crystals of the same mineral connected at precise angles. The angles are controlled by the crystal's atomic structure. For example, in a common type of quartz twin, such as this one, the crystals join at 84° angles.


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits