In my considered opinion - grading can only be learned by in-hand experience from a person that is knowledgeable. This is why attending shows and spending time with other collectors is valuable. Tying to learn accurate grading from a one dimensional photo with high variable lighting, varying degree of focus and none of the advantages of in-handing examination is like trying to learn swimming from watching it on TV. The ability to rotate a marble in your hand, vary the lighting, feel the surface and then take all of the factors of wear, sub-surface issues, blow-outs, cold rolls, etc etc and combine them to an overall grade take time and experience. Experience that comes from a wide range of marbles over time.
The same is somewhat similar for accurate marble identification.
The use of web or book-only grading and identification has led to an unfortunate growth of overly optimistic (and inaccurate) grading and misidentification of marble types. There is only so much one can learn from photos. Shows and the broad expertise there is a valuable source for learning.