First, I created my new layers as a duplicate of the background -that way you can add effects knowing how they’ll affect the original picture better.

Second, I selected different areas and used different crystal sizes to make the crystalization look more random/natural. I used a 50 pixel feather between the areas so they would run together more smoothly.

If you did this using a duplicate copy of the background, you now need to desaturate your edges to get the right effect.

Then after finding the edges, I picked out parts of the picture that I did not want as crackled as others. I selected those areas and decreased the contrast while increasing the brightness to fade the cracks into the background.

Lastly, I used an embossing filter on the whole thing to give it a real cracked look. I used a dept of only about 3 pixels.

If you use a low opacity, you can use multiple layers like these to get an even more natural effect.