The dye colors created with metallic salts are responsible for the beautifully harmonious tones that we associate with natural dyes, but they have a “dark side”. Over time, these metals that are within the dyes themselves will begin to oxidize and corrode. When they do, it can break down the wool or silk fibers with them. As this occurs, the rug pile will become brittle and flake off a tiny little bit at a time. When you run your hand over these oxidized areas, the corroded sections will feel a bit rougher and have a lower pile height than the areas around it that didn’t use metals in the dye. This creates a high / low embossed look and feel and will be especially present in the darker colored areas of the design.
Corrosion might occur over the whole rug, but it is seldom evenly worn as different dyes have different concentrations of the metallic elements. That is why you might find that the lower pile corroded areas are only noticeable in certain colors. It is more common in darker colors like brown and black, because those colors require more iron to create. This uneven pile is what gives the carpet that high-low appearance. You will sometimes see white warp and weft threads of the rug’s foundation peaking through in areas of low pile.