If you want to do a first class job, special equipment is required. And you will be surprised to learn that you don't use a microscope! The best micro photos are obtained not by attaching a camera to a microscope, but by attaching a special lens to a camera!
This is a big topic and I have a whole set of notes and pages elsewhere about it. This short note is just an introduction that is intended to get you thinking about it in the right way.
As with most things, there are many ways to do it. It is possible to take surprisingly good photos by just holding a cell phone up to a microscope eyepiece!
Some microscopes are equiped with a "trinocular tube" that is intended to have a camera attached. These often require a relay lens that is missing (being an added cost item that was either never ordered or that was misplaced). These may work out well for photographing flat subjects like printed circuit board. With 3 dimensional subjects such as mineral specimens the business of shallow depth of field becomes an issue.
What the serious micro photographers do is to photograph using a special lens coupled to their camera. This lens is often a high quality microscope objective intended for use on a metallurgical or biological camera. In addition to this, the camera and lens are mounted on a "rail" which allows computer control of focus position. This allows a series of images to be captured at different focus positions. These images are then combined using special "focus stacking" software. I have detail about all of this elsewhere.
Lastly, a vital consideration for the best micro photography is lighting. This is true of all photography, but especially of micro minerals. Usually several lights are used, often with diffusers. Some people use flash with excellent results. It is imporant to consider the color of the light sources and make adjustments acccordingly to get proper white balance and colors.
You can find my notes (not terribly well organized) on all of this using the following link:
Uncle Tom's Mineralogy Info / tom@mmto.org