Naturally perfect gemstones in large sizes with desirable color and high clarity are quite rare. 

For this reason, those high quality natural gemstones command extremely high prices.  You are basically paying for the rarity.  Synthetic stones can be perfect in color, clarity and size, but they are not rare.  That is why they are less valuable.  Selling a treated gemstone as "untreated" is deceptive and unethical. 

But there is nothing wrong with treated gemstones as long as the treatment is disclosed, the stone is priced accordingly, and the changes by the treatment are permanent.  Because of gem treatments, many attractive gemstones become available at affordable prices. 

Ideally the buyers should buy a stone at a fair price knowing what exactly they are getting.  I am writing this guide for those buyers.

A treatment is any process to improve the quality of the stone, i.e., to alter the color, to enhance the clarity or to strengthen the durability of a gemstone. 

Heating is not the only treatment.  Technically "unheated" could mean the stone is treated in another way.  Here are some of common treatments.

Heating:

The heat treatment may lighten, darken or entirely change the color and enhance the clarity by melting the "silk" inclusions.  Most often treated gemstones are ruby, sapphire, amber, amethyst, citrine, aquamarine, morganite, kunzite, pink topaz, zircon, tourmaline and tanzanite.  Heating is usually acceptable, if disclosed. 

Tanzanite must be heated to obtain the characteristic blue/violet color.  It could be by nature like volcanic activity, lightening or forest fire, or by human as a heat treatment.  Unheated tanzanite is unattractive brown.

• An on-line article of the Colored Stone magazine (July/August 08 issue) reported  the American Gemological Laboratory (AGL) and American Gem Trade Association (AGTA) detected cobalt-coating on smaller calibrated tanzanite.  This treatment produces blue color.  Naturally the blue color of tanzanite are not very deep, and smaller tanzanite gemstones are usually pale.  Only good sized stones can show strong color.  These cobalt-coated stones show paler color on the facet edges and culet tip after the coating wears away.    

I have read an article regarding possible heat treatment on red or green andesine.  The thread-like inclusions in those andesines are very similar to the labradorite schiller melted by high heat.  Many red or green andesines show the same orangey red or grayish green shade. That is another sign of treatment.  Untreated gemstones show natural variation, even in a small parcel.  No treatment is disclosed on this stone on the market. 

• Jewelry Television, which has been selling andesine as untreated, recently admitted a heat treatment on this stone (most likely a diffusion treatment with copper and iron; see the listing below), and offered a refund, if  returned, for a short period (as of July 08). For details check www(dot)jckonline(dot)com and search for andesine. (Replace (dot) with .) 

Diffusion Treatment:

As the people in the trade call it "cooked with chemicals", it is a heat treatment with chemicals that produce color.  The color could be only on the surface or impregnated in the stone. Stones commonly with this treatment are blue and orange sapphires, ruby, and green topaz.  There is no natural green topaz.  Mystic topaz is coated with iridescent coloring agents.

Lately some of blue topaz are also treated in this way to avoid the irradiation treatment listed below. 

Beryllium (Be) diffusion produces the colors ranging from pink to orange to yellow in corundum including the famous Padparadscha color.  Thailand is the world center of the beryllium diffusion treatment.  Acceptance of this treatment in the trade is still controversial.

• As of May 09, copper diffusion on some African tourmaline has been reported.  This treatment would make common tourmaline test as copper-bearing, Paraiba-like tourmaline, which is much more valuable.  However, the gemological community is still skeptical on the existence of the copper diffusion on tourmaline.  The main reason is the report came from only one gemologist and no one else has not yet confirmed it.    

Irradiation:

In this treatment, stones are exposed to radioactive radiation.  Some people call this treatment "nuked".  Most common gemstones that undergo this treatment are various shades of blue topaz, pink and red tourmalines, and some quartz ("Oro Verde" and prasiolite a.k.a. "green" amethyst).   Untreated blue topaz and pink and red tourmalines get their colors by natural low level radiation over a very long time.

Dying/Staining:

Common gemstones with this treatment are ruby, emerald, jade, lapis lazuli and chalcedony.  Black onyx is a dyed chalcedony.  Dyed lapis lazuli and chalcedony are acceptable in the trade.

Fracture Filling:

Stones with surface-reaching fractures are less durable and appear dull/whitish.  Stones filled with an appropriate substance may have improved durability and better clarity.  Common fillers are oil, wax, glass and epoxy resin (Opticon).  Emeralds are commonly filled with oil or epoxy resin.  Some types of opal are occasionally stabilized with epoxy resin.

Many of rubies lately coming out of Africa are glass-filled.  Thailand is the world center of the glass filling.  These rubies are much cheaper (less than 1/10th of heat-treated rubies) wholesale. 

• The Jan/Feb 08 issue of the Colored Stone magazine reports that lately many low grade rubies and emeralds with severe fractures are flooded on the market, and these stones have been filled with glass or some types of resin as extensively as they are on the border line between natural and man-made.  These stones look nice at a glance.  However, they fall apart upon cleaning even done by the professionals. 
• I see this type of stones on Jewelry TV a lot.  They look somewhat cloudy and give you an impression of deep color.  You may see wavey streaks, when the light hits the surface.       

EBay shoppers, know what you are getting and pay a fair price both for you and the seller.  Sellers normally advertise a high quality item as high quality.  But deceptive sellers advertise a low quality item as high quality, but never oppositely.  Remember no one sells truely natural high quality stones at $0.99.

This guide is updated from time to time, when more information becomes available.  If you learned something new and useful from this guide, please click "yes" below.  Thank you!!