Quartz Family of gemstones and crystals: Amethyst, Citrine, Clear Quartz, Smoky Quartz, green quartz
often heat treated, sometimes irradiated sometimes dyed
most often heated to clear up internal inclusions and change the general color.
For example, Citrine is almost always heat treated amethyst or smoky quartz.
Odd colored quartz such as dark smoky quartz or greenish quartz, is mostly irradiated. Chalcedony Family of gemstones: black Onyx, blue chalcedony, Agate, Jasper, Carnelian, Chrysoprase, Bloodstone
All of these gemstones are also under the quartz family. Some of them are most often dyed to obtain a different color than natural, for example the black onyx commonly seen in jewelry is almost always dyed agate.

Chrysoprase is a beautiful natural green material quite prized but it still can be enhanced through the dye process.

Beryl Family of gems: Emerald, Aquamarine
Emeralds - oiled - done for centuries to fill cracks and make emeralds appear better than natural, also fracture filled with green dye high tech polymers a more permanent than oiling but still not absolutely permanent. More common in the last decade, also heat treated to remove inclusions of rutile which gives the stone a hazy look. It is almost impossible to purchase an all natural emerald these days, fine untreated nearly flawless larger size emeralds command a premium price.

Aquamarine - almost always heat treated to bring out the beautiful blues this stone is known for. But it can also be irradiated.

Tourmalines - Rubellite ( ruby red ), Indicolite ( blues ), Greens, Oranges, Browns, multi colors , clear, black
It seems as though the majority of tourmaline gems ARE NOT treated or enhanced to my knowledge.
My pet peeve, is that many dealers try to sell bi-color ( two color green and red stones or crystals ) as Watermelon tourmaline. While bi-color tourmaline gems and crystals are indeed beautiful in their own right , BI-COLOR DOES NOT = WATERMELON !! Bicolor is a cut stone or natural crystal with one part green and one part red/pink. Watermelon is always a stone or crystal with the inner part showing red/pink and is surrounded by green. Watermelon crystals are always sliced like a loaf of bread to show this watermelon effect. In my opinion watermelon crystals and polished stones are much more rare than two color gems and crystals and command a premium price . This is why I believe that dealers want to sell bi-color tourmalines as watermelon tourmalines, so they can get a higher price.
Other color combinations of tourmaline also exist, like green and blue bi color or red and orange or brown and black.

Topaz family: Blues, peach, pink, salmon, reddish orange, bi colors, greens
Almost all the different blue hue topaz is irradiated, there is some naturally occurring blues but they are usually quite pale blue and not the fantastic blues offered by most jewelry dealers. I would say that many of the peach /pinks etc. are either irradiated and or heat treated to get the desired colors. Also in the last few years, there have been newer ionic treatments such that an atomic layer of material is fused to the outside skin of topaz and other some gemstones to alter the color. We have seen green topaz with this fusion treatment.

Corundum, Sapphires and Ruby
Almost all ruby and sapphires are at least heat treated prior to sale to the public. In the last few years it has been detected that rubies have been also fracture filled with dyed high tech polymers and has caused quite a stir in the trade as it was not generally known in the business. Like emeralds all natural non treated/filled rubies command a premium price. It is quite common for ruby and sapphire to undergo heating to get rid of rutile inclusions which cause a hazy look to the stones. You may not know this , but rutile inclusions also cause a cat's eye effect in many gemstones including ruby, sapphire, tourmaline, quartz, and kunzite among many other gem varieties. For the rutile to cause a cat's eye effect it must be concentrated in close parallel bundles along one axis of the crystal/gemstone. Other wise the random rutile inclusions just cause haziness.

Diamonds
Mostly diamonds are just cut and polished, but colored diamonds such as greens and blues, yellows, and pinks and reds, are quite rare in nature and so many of the colors offered in the market have been irradiated and heat treated to obtain the desired colors. Two other things are done to increase the beauty of naturally flawed diamonds, one is fracture filling with high tech polymers, and the second is using a laser to drill micro holes to get rid of black spots in the middle of diamonds. Both processes must be disclosed to the consumer prior to the sale.

Howlite
Natural howlite is generally a white body with gray streaks spider webbing through the material. It is a beautiful natural material and is easy to carve in to any shape. Howlite is listed here because it seems the number one use of this material is to make simulated turquoise. Howlite readily takes dye and can be made to resemble many other gemstones beside turquoise, like lapis and others. The seller should always tell you that you are buying a simulated item to be fair. It is not the worst thing in the world to buy a simulated article, some times all natural items are very pricey and the simulated materials might be the only thing that fits in your budget, but you should know that you aren't buying a 10 inch tall turquoise Buddha statue for $50.00 when in fact you are buying a dyed howlite statue that looks like turquoise. Always remember one universal truth, if something seems too good to be true, it's most often not true.

Jade - Jadeite - Nephrite
There are two varieties of jade the more common is nephrite, and the more valuable is called jadeite. In the west these are the only two materials factually and accurately called jade. In parts of Asia and China it seems that there are many things that are called jade, this is why you should know a little about this prized gemstone so you don't get taken. It seems that every wannabe dealer on web sites such as Ebay call anything green - jade, but now you know the truth. If it's not either jadeite or nephrite, it's NOT JADE!!
A simple steel knife scratch testing can give you a clue about the material. Use a quality steel pocket knife with short blade so you don't end up slicing your hand or finger, find a place on the back or bottom of the so called jade item, and try and scratch a short line in the material. If it's really jade of either variety , you will not be able to scratch it or raise a powder. Most jade look a likes are soapstone or some other dyed material and in general are much softer than either jade, and you will easily be able to scratch it with a short steel knife point. Soapstone carvings can be beautiful in there own right, but you should know that is what you are buying and not be deceived by someone offering a jade statue or necklace.

Lapis Lazuli
Beware of dyed materials that are made to try to look like lapis, for example dyed howlite. Some lapis is really lapis but has been enhanced by dyed to a deeper blue to look like a higher grade. Also some lapis has been treated with high tech dark blue polymers. There is some beautiful royal blue lapis on the market, but it usually commands a premium price.

Turquoise
Natural turquoise is often too unstable to be used in jewelry so it is often subjected to a stabilizing treatment that soaks in various binders such as polymers to add strength. This is a common trade practice and in my opinion should not detract one from buying specimens or jewelry containing stabilized turquoise. Be aware of simulated turquoise ( see Howlite above ), and so-called re constituted turquoise. Re constituted turquoise is crushed up turquoise that has a binder applied, then re formed in to blocks which are cut in to gem stone shapes of carved in to figurines etc. The dealer should tell you if you are buying re constituted turquoise, it should not cost as much as natural or natural stabilized items.

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