Posted on in Know more about gemstones

Methods of processing gemstones

Did you know that it is possible to play with the color of sapphire? If it is heated, a blue sapphire can have its color lightened or on the contrary accentuated. This is a treatment. Some stones are treated to improve their color, clarity or durability, making them more desirable. Thus, treatments significantly increase the number of good quality stones that can be used in jewelry! Alchimie Paris invites you to discover the different methods of gemstone treatment and which stones are concerned.

There are two main categories of treatments: those affecting the color of the stones and those affecting their clarity. Let's start with the color.

There are many possible treatments to change the color of a stone. The most common is heating. Among the stones frequently heated, the most iconic are sapphire and ruby. In addition to blue sapphires, rubies are heated to intensify their red color by removing a purple or brown tint. Heating of rubies and sapphires is so common that their value is based on the heated stones - good quality unheated stones are more expensive. Most tanzanite, zircon and paraiba tourmaline is also heated.

Maison Alchimie Paris - Les traitements des pierres - Rubis chauffé

Heated ruby

Some stones are exposed to electromagnetic rays that will change their color: this is called irradiation. Blue topaz in particular is almost systematically irradiated.
Other treatments exist and are considered to be much heavier: these are diffusion and beryllium treatments. Diffusion treatment consists of heating a stone to a very high temperature to allow chemical elements to penetrate below its surface, giving the stone a new color. It is a surface treatment, so care must be taken: if the stone is repolished, the color may be partially or totally removed! Beryllium is a special case of diffusion used for some yellow or pink-orange sapphires. Its particularity is that it penetrates much deeper into the stone and is therefore more stable.

Spinelle traité par diffusionCobalt diffusion treated spinel. On the right, a slice of another
spinel treated by cobalt diffusion.

The last two possibilities consist of dyeing a stone or depositing a colored substance on its surface (coloring). The value of stones treated in this way is much lower than that of untreated stones, with the exception of onyx - obtained by dyeing an agate black.


Natural chalcedony (left) and cut and dyed chalcedony (right).

Let's move on to treatments that affect the clarity of gemstones, i.e., the presence or absence of inclusions or cracks in the stones.
The most common treatments are impregnation and oil. Impregnation involves filling the pores and open fractures (also known as "frost") of the stones with a substance. Oil is used in particular to treat emerald. Indeed, emerald is usually very included, with many frosts. The oil allows to close them and thus to make them much less visible. Like the heating of rubies and sapphires, the vast majority of emeralds are oiled.

The oil and wax used for impregnation are colorless. Impregnation can also use colored substances, especially to treat opaque stones like jade or lapis lazuli.
Finally, the frost of some stones can be filled by injecting glass during a heating process. This treatment is used on ruby and spinel. Diamond is a very special case, and some diamond inclusions are removed by laser.

For a quick overview of the treatments practiced, you can refer to this table:


Treatment Stones concerned
Heat Ruby, sapphire, tanzanite, zircon, paraiba tourmaline, aquamarine, citrine, spinel, morganite, topaz, kunzite
Irradiation Topaz, morganite, tourmaline
Diffusion Ruby, sapphire, spinel
Beryllium diffusion Yellow/pink-orange sapphire
Dyeing Quartz (especially agates), jadeite, lapis lazuli, turquoise, opal
Coating Quartz, topaz, tanzanite, spinel
Impregnation Opal, jadeite, lapis lazuli, turquoise
Oil, wax Emerald, jadeite
Fracture filing Ruby, spinel
Laser drilling Diamond


Treatments are generally identifiable by gemologists with appropriate instruments. While most treatments must be systematically declared, this is not the case for heating and oiling, which are accepted on the market. The value of treated stones is often lower than that of untreated stones of good quality. This depends on the type and extent of treatment: a lightly oiled emerald will be worth more than a heavily oiled emerald.

Stones are often treated, and have been for a long time, to make them more desirable. Thus, a treatment is not at all a bad thing in itself! However, it can become a bad thing if it is used as a lure, so care must be taken. 

At Nascendi Paris, some of our beautiful stones are heated or oiled, but only to make the stone more resistant. The treatment is then indicated on the product sheet. In case of doubt, do not hesitate to contact us (by email at or by phone at 01 87 20 13 06), we will answer you with pleasure!

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