A "DOUBLET" is an assembled  or composite gem that is comprised of a top and a bottom, with an  adhesive in the middle to fuse them together. The most commonly seen  doublets today are opals, using an opal front and a black onyx backing  to make the stone seem darker and more lively- (referred to as "play of  color" in an opal). This enhancement also added durability to the gem.  This would probably be the most commonly used assembled stone that a  consumer might be familiar with today. 

Looking at the stone from the  side will reveal the layers. Triplets include the additional layer of  glass or a clear colorless quartz cap, a dome on top, to protect and add  dimension. If the stone is mounted and the mounting obstructs the sides  or back of the stone, it can be difficult for the inexperienced to  positively identify a doublet or triplet assembled stone. 

Doublets were also once used  in Victorian (mid to late 1800's) jewelry, what are known as "garnet  doublets" to imitate ruby, sapphire, emerald and other colored  gemstones. Wondering how a green stone, for instance, can come from a  red garnet and glass? 

JUST BECAUSE THE GEM SHOWS INCLUSIONS UNDER MAGNIFICATION DOES NOT MEAN THAT IT IS NATURAL!!! This  is how the magic happens! The top, (crown) was garnet, the bottom  (pavilion) was green, blue, or red glass, an optical illusion, only the  color of the bottom layer shows- the garnet provides no color. Again, to  identify, either look at the side of the stone through high  magnification or immerse in a liquid that has the same refractive index  as garnet (4.0) and a red "ring" or halo will appear through the liquid. 

A  group of simulated blue sapphire garnet and glass doublets.. table side up.

The same group of simulated blue sapphire garnet and glass doublets.. pavilion side up, see how pronounced the red halo is?

Another grouping of simulated blue sapphire  doublets. These have a clear layer, thus, no red ring. But they are  still assembled doublets. Table side up. 

The same grouping, pavilion side up. These are  doublets, but not garnet and glass, so no red halo. But if you look at  them very carefully under high magnification from the girdle side, you  will see the line of separation and a "clear" line where they are  assembled. 

All 4 of these simulated blue  sapphires are doublets. Only the small round one is a garnet and glass  doublet. The other three are doublets also but not garnet and glass....  Table side up. GENUINE portions of sapphire may also be used, this is  where it gets very confusing. An extremely thin layer of genuine  sapphire may be used for only the table facet. If you look closely where  the facet junctions meet, you can see the line of separation. You may  also see a ridged indent along the line of the girdle. 

The same 4 doublets,  pavilion side up. Note the  garnet and glass "red halo" on the left. the other three use different  materials but are still doublets. Genuine sapphire may be used for only  the table facet, and the line of separation is noticeable with high  magnification. The confusion lies with the fact that a cursory  inspection reveals "inclusions"- but the deeper layer will reveal  bubbles or gas, or swirls (glass). These are the most difficult to  identify. JUST BECAUSE YOU SEE INCLUSIONS DOES NOT DISQUALIFY IT AS A  POTENTIAL DOUBLET!!!

Garnet and glass doublet which simulates a green emerald.

If you look very closely you will see the line where the garnet and glass meet, also notice the natural inclusions of the garnet, and the gas bubbles of the glass.

Here is the ultimate corundum  conundrum! Natural blue sapphire table, with synthetic sapphire  pavilion. The giveaway is the deep ridge line on the girdle, and the  curved striae lines of the synthetic sapphire (think of the groove lines  on an old vinyl record). Genuine sapphire will always exhibit STRAIGHT  growth lines, never curved. See how the lines on this bend slightly?  Also, this easily will fool particularly if a stone like this is in a  mounting. Add to that the refractive index reading that one would get on  a refractometer (reading for that of sapphire) and you can grasp the  confusion, even for a gemologist!

A rose gold Victorian 10K garnet and glass doublet ring with seed pearl accents. The doublet in this ring simulates peridot. Garnets  were used as the top surface layer because of their hardness and  durability. Typically this process was used on faceted stones, but  occasionally used on cabochons (un-faceted half round stones with flat  bottoms). 

10k gold antique Edwardian stick pin with garnet and glass doublet that simulates a peridot in a clover shamrock setting. 

The doublet in this ring simulates emerald. Antique Victorian 10k gold garnet and glass doublet with seed pearls ring. 

The doublet in this ring  simulates blue sapphire. Doublets would have been predecessors to the  synthetics that were about to come to the market place in the coming  decades. In 1902, synthetic corundum was invented by French chemist  Auguste Verneuil, making the need for doublets "old technology". 

Another doublet that imitates  blue sapphire. Doublets made use of thin pieces of rough garnet in  order to imitate fine gemstones. This brought a variety to the market  place, since the precious gems would not have been affordable to the  masses. 

 Antique Edwardian 10k gold  wire strung pearls and blue sapphire doublet lavalier pendant necklace,  circa 1900. The really intensely saturated "cobalt" blue sapphires  should always raise suspicion. 

Tiny antique Edwardian 10k  gold blue sapphire doublet good luck wish bone pin. Even REALLY tiny  gems could be doublets, although this isn't typical. 

 14k rose gold Victorian heart shaped pearl and sapphire doublet lavalier pendant, c1900.

The most commonly seen colors of doublets include green  garnet and glass doublets that resemble emeralds,  blue garnet and  glass doublets that resemble sapphires,  and occasionally red- resembling a ruby. 

antique Edwardian rose gold ring with garnet and glass doublets, signed Ostby & Barton (Providence, RI) circa 1910

Rose gold garnet and glass doublet with pearls, antique Victorian signed Ostby & Barton (Providence, RI), circa 1910. 

If you own a doublet in your antique jewelry collection, you  will have to be extra careful cleaning it. I wouldn't recommend using an  ultrasonic cleaner, the vibration is too strong and could actually  separate the layers. Use a really mild detergent like Dove Liquid, warm  never hot water, and a very soft brush (a baby toothbrush is great).  Rinse well and gently pat dry. We also love "canned air" to dry our  jewelry. 

A few possiblities...