Generally, jadeite dyed with chromium salt will turn red under Charles filter, but now many jadeites do not use chromium salt. Some stains are invisible with a filter. Charles filter is not a brand. I usually buy these things at jewelry exhibitions. You can come to Beijing to buy it when you wait for the Beijing Jewelry Show. Or there is a jewelry street in Tianjin, just near binjiang road (that is, the alley near the pedestrian street)