First eat two cloves of garlic. Next do a shot of cheap tequila (no lime). Now breathe across the coin and take a picture through your breath. BTW - This doesn't work if you're wearing an N-95 mask.
Actually, it is negative image, then gamma correction adjusting the red/green/blue spectrum until you get a little more detail and contrast. In this case, I did a negative of the negative after gamma correction. It was a deep purple hue before. The lighter green has more contrast. All done in an old version of Paint Shop Pro.