The Diamonds
There are three classifications of diamonds used on diamond bits:
1. Single Crystal (West African-Bortz): These diamonds are
generally translucent, shiny and come in geometrically regular
shapes, such as octahedrons, dodecahedrons, and other shapes
tending towards spheres.
2. Coated (Congo): These diamonds have a heavy surface coating
or skin which is usually greenish or grayish in color, and does not
permit the transmission of light. They are balas (rounded) in
shape.
3. Carbonado (Black Diamond): So termed because the majority
are black in color and do not transmit light. The majority of these
diamonds have a non-crystalline or amorphous structure.
Diamonds used in oilfield bits are of natural origin and range from as small
as 15 stones per carat to as large as seven carats per stone. Diamonds are
resistant to abrasion, extremely high in compressive strength (the hardest
material known) but are low in tensile strength and have high thermal
capacity. The low tensile strength reduces its ability to withstand impacts.
The terminology used to describe diamond quality is quite varied, and
“quality” is roughly defined by the following factors:
1. Surface Condition: A glossy, smooth surface denotes a surface of
better quality.
2. Translucence: In crystalline diamonds, the ability to transmit
light is indicative of higher quality. This is not necessarily true
when non-crystalline diamonds and coated diamonds are being
evaluated.
3. Internal Structure: The absence of large internal fractures,
inclusions, and growth structures are indicative of high quality.
4. External Shape: A block-shape or nearly spherical diamond is
stronger and hence of higher quality.
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