Does the mat produce FIR rays when not plugged in? If so, how does it compare to when it is plugged in?

Same as with ions, the crystals generate FIR rays all the time. 

When the mat is unplugged, they use the heat of your own body. 

The body heats up the crystals which then emit rays in so called “crystal” wavelengths, between 4 and 15 nm. 

It is the wave range best utilized by the human body, which is also called a “life” or “bio-resonant” range. 

amethyst concentrates most of its heating energy on the two peaks at 7 and 12 microns.  

So it takes from the body (or from an electric heating system) the wide wavelength range heat and returns it in the form of isolated heat in this narrow range with 2 peaks. 

That is why it believed to be a “superconductor” of FIR rays and have powerful healing properties.

Of course, the more warmth crystals receive, the more they emit. 

So whether you heat up your mat with the sun (like in ancient times) or with electricity, it gets really hot and emits more energy. The wavelength is the same. 

It does not mean that you should always use the mat on the highest heat. 

Unpowered or low temperature ranges are much safer and more gentle. 

You should take care not to overuse the mat, especially if you sleep on it or use it for longer sessions, because our body’s temperature sensors do not feel penetrating radiant heating in the same as they feel something hot to the touch.