Updated: Apr 22, 2021
This is a photo of a natural on the girdle/pavilion of a diamond. So, what did I just say? Well, a natural is the original (or natural) skin of the diamond crystal left intact during the cutting process. Sometimes, the natural follows the faceting line, which is referred to as just a natural. Sometimes, it pushes into (or indents) the facets, resulting in an indented natural that is sometimes mistaken for a chip without magnification.
In this case, we see a few things, one of which is called a trigon. See that perfect equilateral triangle? I suspect some of the lines above it were once also trigons, but that perfect little triangle tells us a few things. One, this is definitely a natural and not a weird extra facet, and two, it is an earth-mined diamond.
Often, if you have a natural on one side of the girdle, there will be one directly opposite because the cutter used as much of the crystal as possible to achieve the final faceted diamond.
This is the natural directly opposite the above. It's not as pronounced, but it is a natural and shows that as much of the crystal was used to obtain this round diamond.
Sometimes naturals are placed under a prong and can appear like a chip. This is an indented natural from the same stone-- the cutter really got everything he could from this crystal-- and unlike the other two naturals, this one does break the facet line.
Naturals are clarity characteristics and are great ways to identify your diamond even if there are no significant internal inclusions.
Of course, I am always happy to answer questions!