Gemstone Treatment Frequency Remarks  
Diamond Laser Drilled Sometimes Diamonds are laser drilled to bleach out  dark inclusions.  
Diamond Clarity Enhanced Sometimes Inclusions breaking the surface of the gemstone can be masked by placing a glass like substance in the crack.  The substance tends to mask the location of the crack.  
Emerald Oiling Usually When observing an emerald it is best to assume the gemstone has been oiled. 
Emerald Epoxy like filler Often Many emeralds are treated with an epoxy like filler.  The filler flows into the cracks on the surface of the gemstone.  The filler tends to mask or hide the location of these cracks.  
Sapphire heated Usually Most sapphires are heated to improve the color of the gemstone or the uniform distribution of color within the gemstone.  
Ruby Heated Usually Heated to improve color  
Ruby Dyed Rarely Dyed with colored oil to improve appearance  
Ruby Heated to seal fractures Often Rubies are heated in a controlled environment in a borax solution.  The borax can heal fractures in the surface of the stone.   Occasionally the borax residue can be seen under 10 power magnification.  
Ruby Clarity Enhanced Occasionally Rubies are occasionally Intentionally treated with a substance that  fills surface cavities and fractures with a foreign material. This filler can have glass like properties  
Amethyst Heated Occasionally Amethyst is sometimes heated to lighten the color or remove a brownish component  
Citrine Heated Usually Amethyst is often heated to lighten the color to a yellow or yellow brown  
Tanzanite Heated Always Zoisite (Tanzanite) is typically a yellowish brown color when mined.  The gemstone is heated to produce the bluish purple to purplish blue color the public prizes  
Blue Topaz Irradiated Always The gemstone is bombarded by neutrons to produce the blue color.  
Pearls Bleached Usually Cultured Pearls are usually bleached to improve color.  
Pearls Dyed Occasionally Black or gray pearls are typically dyed to these shades.   Rose or blue tints are sometimes the product of dying.  Freshwater pearls in a variety of colors are usually dyed.  
Pearls Irradiated Occasionally Pearls are occasionally irradiated to produce black or gray colors.  
More to follow at a later date..   If you have a particular question about a gemstone