Help creating 3d image with background features removed (image example in post)

I’m pretty much a complete noob. I have CT-scans of rock cores, roughly 1000 tiff images. I’m trying to analyze density as well as produce images showing the higher density (lighter colored areas) areas in the rock core.

Here is an example of what I’m trying to do.

I can load the image and crop and all that very basic stuff, but after that I’m lost.
Any help would be much appreciated.