Composite or assembled stones are made by cementing two or three pieces together to produce a single stone. In general, when two pieces are cemented together, they are known as doublets and when three pieces are used, they are known as triplets. Irrespective of whether the stone is a doublet or a triplet, it is essential to be aware of their existence and also to know the means to identify them.
Composite stones are made for the following reasons:
- To improve colour and appearance.
- To improve phenomena.
- To improve durability (surface lustre or base support).
- To gain weight.
- To imitate any other gemstone.
Identification: Certain standard guidelines must be maintained while identifying composite stones.
- Lustre: Varying surface lustre in some cases.
- R.I. for two opposite surfaces i.e. table and pavilion facets.
- Immersion: Composites must be examined immersed in a liquid, preferable with additional magnification.
- Two or more distinct parts are visible i.e. varying relief.
- A separation junction plane is seen.
- Inclusions will vary in the two parts e.g. natural and synthetic inclusion.
- Flattened gas bubbles along the junction plane may be visible.
- Coloured cementing material when present.
- U.V.: Varying fluorescence in different portions.
- Polariscope: Optic axis direction in the two portions may vary.
Composite stones are classified into categories using different parameters. One system employed, stresses the nature of the gemstones used as is seen in the following examples.
- True Doublets: Two pieces of the same species but of different qualities e.g. ruby on ruby, opal on opal.
- Semi-Genuine Doublets: Here the crown portion is made of the natural gemstone being simulated while the pavilion is generally a simulant, synthetic or imitation e.g. Diamond on synthetic white sapphire, opal on black onyx.
- False Doublets / Triplets: Here the crown and pavilion is generally colourless quartz or glass with a coloured cementing material or glass in the middle. Such composites can simulate almost any natural gemstone.
Another form of classification makes use of any one specific optical property such as R.I., as a standard. In this case a systematic grouping can be done by considering the R.I. ranges for the crown or table or upper portion of the composite stone.
Commonly observed composite stones.
- Opal: Composites are generally made:
- To provide a support to thin slices of opal.
- To enhance the play of colour.
- To gain weight.
- Opal Doublet: Opal slice backed by black chalcedony, black glass or potch opal. The cementing material may be black or a dark coloured substance.
- Opal Triplet: A thin opal slice is covered by a rock crystal dome & backed by black chalcedony. The opal slice may be natural or synthetic.
Identification is easier if the stone is immersed in water to observe the junction plane. Care should be taken not to mistake a layer of natural opal matrix which is left while cutting, for a doublet or triplet.
- Quartz: Since quartz is widely available this is commonly used in composites. Originally, rock crystal on rock crystal cemented with a green coloured gelatine was popularly sold as soude emerald (from the French – soldered emerald) as an emerald imitation. Nowadays, both quartz doublets and triplets are available and serve to simulate most gemstones.
- Beryl: Composites are made using either colourless or light coloured beryl with or without coloured cement. Commonly they are made to simulate aquamarine and emerald.
- Jade: Triplets are made from three pieces of translucent white jadeite. The top portion is a hollow cabochon, the middle is a dyed green jadeite cabochon which fits into the hollow cabochon and the base is an oval cabochon. The entire piece is then re-polished to eliminate traces of the joining planes. In set jewellery these cabochons are not very easy to identify.
- Spinel: Triplets are made using colourless synthetic spinel for the crown and pavilion, and a coloured gelatine layer or coloured glass cemented in the middle. A wide range of colours is produced and therefore most gemstones can be simulated.
- Garnet: Garnettopped doublets are some of the oldest known doublets.
- Almandine garnet has the ability to fuse with glass which eliminates the need for a cementing material.
- Though the almandine slice is red, all possible colours can be got by using different coloured glasses.
- These doublets can be deceptive since the junction plane can be in any orientation and not always as the conventional plane parallel to the table facet.
- An additional identification technique is the red-ring test, which is observed by placing the stone table down on a white background.
- A white light source is held directly above the culet and a shadow or a red ring falls on the white surface.
- This may not be visible in the case of red or violet stones. The red flag effect may be observed while an R.I. is taken on the table facet and is seen as a shadow on the refractometer scale.
- Corundum: These are some of the most common doublets which are available. Some possible combinations are listed below:
Crown Natural Green Sapphire Overall color is blue. Pavilion Synthetic Blue Sapphire Crown Natural Green Sapphire Overall color is red. Pavilion Synthetic Ruby Crown Synthetic White Sapphire Overall effect is colorless. Pavilion Strontium Titanate
- Foil Backed Stones (Chatons / Rhinestones etc.): These are stones which have a coating on the back facets. The coating may be a foil or a non-transparent material which basically improves the brilliance, colour and / or phenomena in a stone
- E.g. rhinestones are foil backed glasses made to imitate diamond.
- In gold jewellery, stones are foiled to project a better colour.
- Star forms are engraved on a foiled base of a cabochon to appear as star gemstones.
- Multi-colored backing on a glass is done to imitate the play of colour in an opal.