Chemical treatments are often called impregnation because the chemicals usually penetrate the surface of the stone. Porous stones, like turquoise, are frequently sealed with wax or resin to keep the color from fading. Oils, waxes, and plastics are used on many stones to hide small scratches and surface flaws. Coral, ivory, and pearls may be bleached. Nearly all gemstones can be dyed or stained.
Some chemical treatments are permanent.
Others will dissolve in solvents like acetone or in ultrasound cleaners. Waxes melt when exposed to heat or strong light. Bleaching can be impossible to detect. Dyeing can usually be detected with a microscope. Assembled stones may be composites like doublets and triplets, or they may be foilbacks.