Diamond is the Hardest

Diamonds have been treasured as gemstones since their use as religious icons in ancient India. Their usage in engraving tools also dates to early human history. The popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns.

Diamond is the ultimate gemstone, having few weaknesses and much strength. It is well known that Diamond is the hardest substance found in nature, but few people realize that Diamond is four times harder than the next hardest natural mineral, corundum (sapphire and ruby). But even as hard as it is, it is not impervious. Diamond has four directions of cleavage, meaning that if it receives a sharp blow in one of these directions it will cleave, or split. A skilled diamond setter and/or jeweler will prevent any of these directions from being in a position to be struck while mounted in a jewelry piece.

 
The most familiar use of diamonds today is as gemstones used for adornment, a use which dates back into antiquity. The dispersion of white light into spectral colors is the primary gemological characteristic of gem diamonds. In the 20th century, experts in gemology have developed methods of grading diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the 4Cs, are now commonly used as the basic descriptors of diamonds: these are carat, cut, color, and clarity. A large, flawless diamond is known as a paragon.

In terms of its physical properties, diamond is the ultimate mineral in several ways:

1. Hardness: Diamond is a perfect "10", defining the top of the hardness scale, and by absolute measures four times harder than sapphire (which is #9 on that scale).
2. Clarity: Diamond is transparent over a larger range of wavelengths (from the ultraviolet into the far infrared) than is any other solid or liquid substance - nothing else even comes close.
3. Thermal Conductivity: Diamond conducts heat better than anything - five times better than the second best element, Silver!
4. Melting Point: Diamond has the highest melting point (3820 degrees Kelvin)
5. Lattice Density: The atoms of Diamond are packed closer together than are the atoms of any other substance
6. Tensile Strength: Diamond has the highest tensile strength of any material, at 2.8 gigapascals. However, that does not quite translate into the strongest rope or cable, as diamond has cleavage planes which support crack propagation. The strongest ropes can likely be made from another material, carbon nanotubes, as they should not suffer from the effects of cracks and break. Still, if a long, thin, perfect crystal of diamond could be manufactured, it would offer the highest possible pulling strength (in a straight line - don't try to tie it in a knot!)
7. Compressive Strength: Diamond was once thought to be the material most resistant to compression (the least compressible). It is the material that scientists use to create the greatest pressures when testing matter. However, the rare metal Osmium has recently been shown to be even less compressible (although it is not as hard as diamond). Diamond has a bulk modulus (reciprocal of compressibility) of 443 GigaPascals (GPa). The bulk modulus of the metal osmium has recently been found to be 476 GPa, about 7% greater than diamond.