Set the camera to macro, find a piece of white paper (the whiter the better, not transparent) and put it under your desk lamp (preferably an incandescent lamp with a higher color temperature, mine is 5500K). Haha), and then set the white balance (see your instructions on how to set your camera). Let the lamp head of the desk lamp be perpendicular to your stone as far as possible, which will reduce the shadow.

Put your camera on a tripod (it doesn't matter if the light of the desk lamp can give you a faster shutter speed). In fact, the method of photometry is not very important (it is best to measure your stone by spot photometry). The most important thing is that you can make good use of the photometry method you use. If you see bright light, then use exposure compensation to subtract EV. If it is dark, add EV.