SALT & PEPPER DIAMONDS, WHAT ARE THEY?
Diamonds are formed deep within the earth - about 100 miles or so below the surface. A combination of high temperature and high pressure is necessary for a diamond to form; the diamonds we see at the earth's surface are brought up from the mantle by a violent volcanic eruption. By moving quickly from the upper mantle to the surface of the earth (essentially moving rapidly from a high to a low temperature), the carbon atoms are locked in place into a diamond structure. (Smithsonianmag.com)
Black speckles present in a "salt & pepper" diamond are also pure carbon. Just like how freckles can form on the skin as a result of an overproduction of melanin, black speckles of carbon in a diamond simply didn't crystallize at the same rate as the rest of the atoms for one reason or another.
Other diamond colors like orange, green, blue, etc. are attributed to other elements present in the diamond. Orange, for example, means the nitrogen atoms have grouped themselves in a very specific way. Green diamonds were exposed to some form of radiation during the stone's formation and journey to the Earth's surface. Rare blue diamonds are created when boron is captured in the structure. (Scientific American)
The natural black carbon inclusions do not inherently make the diamond weaker - A high quality salt & pepper diamond is the same strength as a white diamond and is every bit as durable.
With that being said, just like with all white diamonds there is a huge variety of quality among salt & pepper diamonds. The really low quality material will have a lot of surface pits, cracks, etc that do affect the structural integrity of the diamond. A lot of times, the lower quality salt and pepper/colored diamonds will be marketed as 'rustic' diamonds to pass off the surface and structural issues in clever marketing terms.