What Diamond Crystals look Like

Interpenetrating cubes of diamond
Photo by Rob Lavinsky

The word morphology means what it looks like.  Diamond crystals are a good example of this as in the growth process they are able to take on many guises.  They all however are caused by the basic shape of the carbon cell that controls the diamond growth.  The carbon cell is used to build the diamond crystal or stated in another way a diamond molecule.  These carbon cells are deposited onto the nucleus of the diamond one layer at a time until the diamond finishes growing.  In its purest form a diamond crystal is cubic like a crystal of sugar.  The addition of various elements however causes a diamond to morph into different shapes or colors.

The most common of these shapes are the cubic form that is square like a salt crystal or the octahedron which is two pyramids stuck together base to base.  From this simple crystal form the shape of a diamond becomes more complex.  This is caused by many different physical process; the most common being the physical space allotted to the diamond crystal to grow.  If this allotted space is cramped and not shaped like a typical space the diamond crystal will grow into the shape of the space.   Something that will further complicate the final shape of the diamond crystal that is delivered to the surface of the Earth is the dissolution of the diamond in carbonate or silicate bearing water at the pressure-temperature conditions encountered in the diamond stability field.  This was established experimentally by Alexander F. Khokhryakov and Yuri N. Pal.Yanov of the Siberian Branch of the Russian Academy of Science in Novosibirsk Russia. 

The macle another form of diamond crystal
Photo by Rob Lavinsky

One of the stranger forms of diamond growth is exemplified by the rough diamonds from the Ekati, Diavik and Snap Lake mines in Canada.  These diamonds are dug out of the ground wearing fur coats that are fibrous overgrowths of diamond crystals with a transparent diamond crystal at the center.  Apparently somebody told these diamonds they were coming from Canada, so they came prepared for the Canadian weather.  How this came about was the diamond crystal had more then one stage of growth. 

There are one or more generations of crystal growth in most diamond crystals.  A diamond crystal can also undergo retrograde growth where conditions either in the area in the mantle where it was formed or on its way to the surface in kimberlite magma.  Some of these crystals can become bizarre in their appearance, and some of the more bizarre are what are called skeletal crystals.  Sometimes the diamonds are etched into more rounded shapes by the action of the chemistry of where they are formed or transported.

The macle is another shape the diamond can naturally take this is a twinned diamond taking a triangular shape and has two large triangular sides.

References:

Harlow, George E., The Nature of Diamonds, American Museum of Natural History, © 1995