Hello :)
I have been spending my free-time exploring and trying out custom materials that I have always hoped to find in KeyShot but never could. I've noticed that when it comes to marbles and bronzes (especially for sculptures and figures), there just aren't that many available that really look that good. So, I decided to try and work on some of my own, using only textures provided by KeyShot.
I'm really enjoying the learning process, and it's really fun when I feel like I'm getting closer to realistic looks and feels. The models are not mine - I used them for educational purposes from ScanMyWorld and ThreeDScans.
Some of them I know I need to make less specular, and others could use a little more roughness/bump, but I think it's a nice start :)
I spent some time on a marble material myself not too long ago (fifth image (https://www.artstation.com/artwork/Y193w)). I think you're third image is pretty good. In my experience, you really need a quality marble / granite texture to bring out the color variation you normally see in marble. Also, fresh marble and aged marble look much different, with the latter typically having various dust layers of different strengths and colors.