How to segment rocks from the given image?

Since background and rock are having the same color, I needed to segment the given image by some means. I tried using rgb and hsv processing but so far got this only, now how to remove small marks and dents and create a boundary to cover segment each rocks with some particular size. Tried using bwperim also but didn't worked out :( Here is the image and here is the image I segmented.

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Answer 1

[Image Analyst]

See granulometry click here