Sentence:  There are a very large number of types of <WSD>blemish</WSD> and the smallest <WSD>blemish</WSD> visible to a human can be surprisingly small, for example less than 10μm deep, which may be on the surface of a heavily embossed tile. 
Meaning of the word marked with <WSD> tags:  A small flaw which spoils the appearance of something, a stain, a spot.