I own a cheap RFI detector. Used mainly for AC power leaks.
I find if I coat small quartz crystals with dielectric grease (like a car motor stuff) Then place the crystals in a baggie and press them down tight, the baggie of crystals totally block RFI from an AC outlet.
If the crystals are left loosely packed, they do NOT block the RFI.
Others use some juice and then have to bake the crystals.. My method with dielelctric grease need no baking.
(though you NEVER want to get dielectric grease in an eye!)
I tried them $100 worth of small various types of white, pink, green, purple brown quartz. I am pretty bored with it though. The result is too small to bother much anymore.
I am just mentioning the blocking effect with packed quartz.