I am finding that one of the secrets to getting interesting photos of crystals, with lots of detail and color, is to grow them thin.  The thicker the layer of crystals on the microscope slide the less light that gets through reducing the colors, and the more depth of field becomes a problem with getting the whole frame in focus.

One thing you can do to promote thin crystal growth is to place a coverslip over the solution on the slide.  This also impacts how the water evaporates which greatly slows the growth process.

The above images were of Epsom Salts grown under a microscope coverslip.

Cross polarization micro-photographs of Epson Salt crystals.  The black backgrounds in these images are the result of adjusting the orientations of the polarizers without the use of a full wave retarder.  The colors in these images are not computer generated, they are the result of how the polarized light passes through crystals of different thicknesses.

(Click on any image to view it full size.)