Taking a good picture of a mineral specimen is a quite tricky business.
Firstly a neutral back ground can be desired.. I.e. black is often good.
Now the camera should be set to 'macro' mode, close up shots.
This is often marked by a flower on the camera.
Its often good to use natural day light, hence some lamps might fool you about the colour especially when a camera is with in the picture.
But a slightly more yellow colour flashlight might work should all else fail.
Often you get 'blurry' pics, not quite sharp detail.
The most common causes are: to close, shaking hands and bad light.
Either is pretty easy to fix...
The shaking hands are easy to just use something to rest the hands on so they can't shake to much.
The light can be solved by using a flashlight or a normal lamp.
Just make sure to check it looks like on the pics.
If your to close, get some distance.
Example of good shots.. Note the rocks are the interesting! Don't bother 'bout the surroundings.

Opal:
Some cabochon I made: