While I am a person that would never buy a diamond with a knot (another crystal, grown at the same time, that is at the outside surface of the diamond ... when they are not at the surface they are called crystals), prior PS advice has been to get it checked by an independent appraiser to make sure it’s unlikely to get knocked out. Often knots near the girdle are not a problem, according to a comment in the below thread ... or maybe that is just for cavities .... anyway, the risk is that the knot gets knocked out and leaves a cavity - I hope you can have the diamond checked before you buy it. https://www.pricescope.com/community/threads/cavity-knot-in-a-diamond-dangerous.28352/