HomeThe nature of Diamonds

16.3 The nature of Diamond
16.3.1 Structure of Diamond

Structure

Diamond is quite unique as it possesses qualities that make it easily stand out from other minerals. It is widely known that diamond is the hardest natural substance known having a Hardness Value of 10 on the Mohs hardness scale of 1-10. One(1) being the softest and ten(10) being the hardest.

This hardness reflects the ability to penetrate or scratch the surface of the gem. Despite being the hardest naturally found mineral, Diamond is rather brittle. That is, it can be ìcrackedî rather easily if it is struck in a direction of its cleavage planes and pieces can be broken off. The difference between hardness and brittleness was confused in early diamond mining in South Africa and many a diamond was crushed or shattered by a miner who use to hit the gems with a sledge hammer to test for a gems authenticity. The belief was if the gem is the hardest mineral it could withstand the blow of a sledge hammer and thus be a real diamond, whereas other substances would be crushed like ordinary rock. Consequently, many authentic diamonds were mistaken for ordinary rock when they split or ìcleavedî upon being hit with sledge hammer or reduced to many smaller pieces.

Another property of diamond is that it is highly thermal conductive. It is extremely effective at conducting heat as a result of the carbon material its formed from and its highly organized atomic structure.

Diamond also has a distinctive feature in its weight, known as Specific Gravity. Specific Gravity is the weight of a material divided by the weight of an equal volume of water it displaces. ìDiamond has a specific gravity of 3.5 which is about a third more dense than typical rock.î Diamonds and the Northwest Territories, Canada pg2. The Specific Gravity of diamond is precisely measured to 3.52 .

Diamond falls into the cubic crystal system also known as Isometric. The mineral itself is a hard dense crystalline form of carbon. Carbon can exist in many forms aside from diamond including the more familiar, graphite, which is found in the common pencil. When carbon is compressed under high pressure and elevated temperatures the conditions exist for it to turn into the form of carbon known as diamond. These conditions needed to produce diamond are found within the earths crust at depths of up to 150km below the surface. Man has also been able to mimic these conditions and has produced man-made diamonds. These man-made or synthetic diamonds are the same in composition however display subtle differences in structure and properties that distinguish them from their natural counterparts. As well when they are created they look quite different than rough diamonds. Its when they are cut and polished do they appear like natural diamonds that are cut and polished.

Each atom of carbon in a diamond has the same shape. The individual atoms of diamond are arranged in an orderly pattern and their shape is reflected in the external shape of the diamond.

This external shape may be displayed in several ways however, the predominant shape seen is referred to as the gems ìhabitî. The predominant habit (shape) of the rough diamond is that of an octahedron. The classic octahedron appears like 2, four sided pyramids stuck together at their base, with the sides of the pyramid on top basically lining up with the sides on the bottom. http://www.cigem.ca/ross/rough.html


16.3.1.1 Chemical bonding
16.3.1.2 Crystalline patterns of carbon atoms
16.3.1.3 Diamond crystal form and habit, growth and etch markings

Octahedron

As mentioned above, the predominant shape of rough diamonds is that of the octahedron.

Dodecahedron

This is also a very common shape of rough diamond. It has 12 faces that give the rough diamond of this shape a less angular appearance than the octahedron. The faces on the dodecahedron as smaller and there are more of them and can give the appearance that the gem ìbulging outî. ìThese (octahedrons and dodecahedrons) shapes are most commonly encountered in rough gem diamonds and represent approximately 50% of rough diamonds in a typical parcel.

Cubes

Diamonds may actually occur in the shape of a cube. This seems natural as diamond is in the cubic crystal system. However, the occurrence of gem quality diamonds in a cube shape is rather rare. ìIt does occur in gem quality form occasionally but much more frequently in industrial (non-gem quality) or non cuttable form.î Rough Diamonds pg 14. The cube surface may be concave or convex as may be the face junctions (ribs).

Whole and Irregular Shapes

Rough diamonds that are modified forms of the octahedron, dodecahedron and cube and display both rounded faces, points and ribs are referred to as ìwhole diamondsî. If the same conditions prevail and the rough is elongated its known as an ìirregularî. These shapes are common to river bed and alluvial placer deposits reflecting the rounding of edges that occur through weathering and transport from their surface origin to the place where their found as a placer deposit.

Surface Features

 

These are features found on the surface of a rough diamond regardless of the shape and are broken down into four categories as follows;

 

Mirror or Glass-like Surface

 

This type of surface can be found on any type of rough diamond and literally provides a ìwindowî that you can see into or through the diamond rough. These types of surfaces are common on the octahedron and dodecahedron shaped roughs. A feature that is often found on the surface of rough diamond, especially those with glass-like surfaces are Trigons. Trigons on diamonds are often quite visible to the naked eye. Trigons look like small triangles that are oriented 180 degrees from the octahedron faces that may be protruding or indented on the surface. They can appear on rough diamonds of any shape and are considered a strong indicator that the mineral is diamond. Image #7 is of a row of trigons at higher magnification. Having said this, the author has observed similar features on other minerals that form as octahedrons such as the mineral fluorite. It is likely that similar feature could be seen on any mineral of the cubic crystal system that forms in the shape of an octahedron. Image #8 is a highly magnified image of the surface of a fluorite in octahedron form that displays a trigon-like form on its surface.

 

Frosted or Waterworn Surface

 

As the name implies these types of surfaces are common to rough diamonds found in Placer deposits especially river bed deposits. These surfaces usually exist on non-octahedral and irregular shaped rough diamonds. This surface is often referred to as a sandblasted finish and displays varying amounts of transparency.

 

Grooved or Serrated Surface

 

These types of surface features are common to all types of rough diamonds shapes and include growth lines, wavy textured surfaces, and step like surfaces.

 

Growth lines are commonly observed at the edges of octahedron faces where the ribs are. As a diamond grows, new layers may grow over the old layers, however, sometimes the growth of the face ends just short of the rib and as such the edges begin to look layered at the ribs.(Image #9)

 

The same feature when observed on less well formed rough diamonds gives a more ìstep-likeî appearance. (Image #10) Rough Diamonds pg 40.

 

A rough diamond that has a ìwavyî appearance on its faces may have a more rounded faces and ribs.(Image #11) Rough Diamonds pg 38.

 

Blemished Surface

 
These features are caused by radiation from rock near the rough diamond that bombarded the diamond and ìstainedî the surface. The degree of staining may be as little as a spot or two, to completely covering the diamond. The staining is usually a dark green in color but may also have an appearance of brownish and grey tones. http://www.cigem.ca/ross/rough.html


16.3.1.4 Cleavage

These are rough diamonds of any shape that have a broken surface that may or may not be in a cleavage direction. The cleavage plane in a diamond is parallel to the faces of the octahedron, regardless of its exterior appearance. Although diamond is the hardest known natural substance it can be relatively easily broken along its cleavage plane. This characteristic is exploited by persons that cut and polish diamonds to easily remove unwanted parts off the rough diamond. Rather than having to saw through the rough diamond, they can ìcleaveî the diamond with tools that basically amount to a hammer and chisel.

When a cleavage plane is recognized, it is a identifying feature of a rough diamond and also lends insight into what the rough looked like prior to having lost a piece along the cleavage plane.

Fluorite is also a mineral in the cubic crystal system that forms as an octahedral with cleavage planes the same as diamond. I used fluorite as it significantly cheaper than diamond and cleaves quite easily. Image #5 shows the octahedral shaped fluorite crystal prior to being cleaved. Image #6 shows the fluorite crystal after being cleaved and the fragment that broke away. Notice the cleavage surface parallel to the octahedron face.

Flats

The term describes a rough that is shallow in depth with little or no body and the shape often does not lend itself to easily being cut into a retail diamond. Typical flats are triangular in shape like the face of an octahedron, semi-rounded to elongated and a often the fragment piece of a diamond that has been cleaved.

Macles

These are odd shaped diamonds that may look like 2, three sided pyramids glued together at the base. The ribs are usually semi-curved or straight and the faces are usually grainy or wavy in texture. These shapes are somewhat of an oddity of nature as the cleavage planes in these roughs run parallel to the base rather than the octahedron faces. These are not commonly used for gem material as they are difficult to cut.

  • Although these shapes mentioned here are among the most common, they in no way represent all the shapes that a rough diamond may occur as.

 


16.3.1.5 Growth planes and grain
16.3.1.6 Symmetry
16.3.1.7 Reference axes
16.3.1.8 Classification of diamond in the cubic system

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