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warshawlawcalled an "artgum". That is probably what you are referencing. They are brown, very crumbly. Most erasers remove layers of paper to eradicate the line. Artgums pick up the pigment as they fall apart rather than wearing down the paper.
Please note that even if the card erases nicely, it is illegal in many states to sell the card without disclosing that it has been altered. The fact that a slabber misses an alteration points merely to the slabber's incompetence.