Twinning isn't a gemstone phenomenon you’ll often observe, as it can only be seen when the material is still in its rough form. But twinning can create some truly stunning shapes that vary wildly from gem to gem, depending on the crystal structure of each one.
The name twinning comes from the way that, very rarely, gemstone crystals can form in pairs. This can occur in different ways, including them forming side by side, intersecting each other or even growing within one another. Scientifically, twinning occurs as gemstones are developing. The result is that their structure may be reflected, repeated incorrectly or rotated, resulting in the creation of a twin crystal that is attached to the original. A twinned crystal will share planes with its twin, and the shape of the stone will often be dramatically affected by the phenomenon. The process of twinning is often triggered by changes in temperature or pressure during the formation of the gem. It can also occur when the raw elements needed to form a gem have temporarily ceased to be available, causing the crystal to pause in its formation and then resume at a later time.
The Green Fluorite shown at the top of this blog is a spectacular example of twinning. Fluorite forms with a cubic crystal structure. You can clearly see how the crystal has formed with two distinct cubes intersecting each other, as a result of an interruption or some other event during its creation. The Calcite specimen shown just above clearly demonstrates how two twin Calcite crystals have grown back to back, though interestingly they feature different colors. This could be an indication that the two twins formed at different times, or that there was more of a particular element present on one side of the formation than the other.
This explanation is a simplification of the complicated science behind twinning, but there’s no doubt that some of the resulting crystals of this remarkable phenomenon are museum quality pieces in their own right.
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