10

Popisky – fyzikální chemie, zkoumání šperků 

Labels – physical chemistry, examination of jewellery

1. The necessity to determine genuineness (origin) often concerns also mineral material. These are counterfeits that can be encountered on the market quite often or these are incorrectly declared materials, for example concerning the origin (location), treatments, etc. It is assumed that 30-50% of materials on the world market are counterfeits or incorrectly declared materials. Forensic experts of the Police of the Czech Republic usually examine this cases based on the request of the law enforcement authorities, the Czech Trade Inspection Authority, etc.

2. In central Europe, counterfeits concerning so called Czech garnet are frequent
"Czech garnet" (chrome pyrope) is a protected regional marking of origin. Only chrome pyropes from a few locations in the Czech Republic can be marked as Czech garnets. Jewellery with Czech garnets is a favourite commodity sold to tourists. Nowadays, the Czech garnet is often substituted with garnets from other world deposits but declared to be “Czech garnet”.

Besides the chemical composition, a garnet´s origin can be detected based on inclusions. Chrome pyropes from České středohoří area are usually rather clear, inclusions are often crystals of apatite, zircon, etc. Zircon often contains uranium or thorium and shows metamict changes which can be often seen in torsion cracks. They are similar to insect wings. These inclusions are characteristic for the Czech garnet from the above mentioned areas.

Czech garnet exploitation in the area of Dolní Olešnice in Podkrkonoší

3. Natural Czech garnet sample

4. Jewellery declared as “Czech garnets”, however, the garnets are not Czech garnet.

5. Necklace marked as “Czech garnet”, however, the garnets are not Czech garnet.

6. Forensic gemmology often sees fakes of natural gemstones made of cubic zirconia (CZ):

• cubic modification of zirconium dioxide (ZrO2),
• Attention - it is not natural zircon (mineral) – zirconium silicate (ZrSiO4),
• density of cubic zirconia is between 5.6 and 6.0 gcm-3,
• hardness is 8–8.5 (Mohs hardness scale),
• diffraction index 2.15–2.18 (for comparison – diamond has 2.42),
• optical dispersion 0.058–0.066 (better than diamond which reaches 0.044),
• synthetic production known since 1929, commercial production since 1976, annual world production estimated over 50 million carats (10 tonnes),),
• CZ is produced also with a thin layer bearing diamond characteristics (diamond-like carbon – DLC) using the process of chemical vapour deposition – CVD, CZ becomes harder, more shiny and resembles a diamond more,
• CZ can be coloured any colour and is used to substitute gemstones (diamond, ruby, sapphire, emerald, etc.) but also cheaper minerals (amethyst, citrine).

7. Synthetic cubic zirconia of different colours

8. Jewellery with cubic zirconia - imitation emerald and sapphire

9. Ring with “a diamond”, however, it is a moissanit imitation of a diamond.

10. Semi-finished synthetic corundum, so called a “pear”. It was produced by melting of Al2O3 and it can be coloured any colour. In former Czechoslovakia, these synthetic corundums were fitted into precious metals and sold as aquamarines, rubies and sapphires.

11. Gold earrings with synthetic blue coloured synthetic corundum which imitate aquamarine.

12. Moldavite is a typical Czech gemstone (tektite)
Tektites are in many places in the world but only those in the Czech Republic are light green. These moldavites are very popular especially in Europe, Southeast Asia and the USA. In the last years, synthetic moldavites produced in China have been appearing on the market. They copy very well both the colour and the typical surface morphology.

Synthetic material – moldavite imitation from China

13. Natural moldavite

14. Synthetic moldavite

15. Sample of natural beryl, its gemstone variety is green emerald or blue aquamarine.

16. Synthetic opal is often to be found on the market:

• chemical composition SiO2.nH2O + resin (up to 20%), (natural opal is SiO2.nH2O),
• hardness of synthetic opal is 4, (natural opal 5.5),
• density 1.80–1.90 (Mohs hardness scale), (natural opal 1.98–2.20),
• temperature resistance 130°C, (natural opal 100°C),
• it is characteristic for its sharply framed colour zones, so called “lizard skin” structure (in natural opal they are diffuse).
  
  Typical, sharply demarcated zones in a synthetic opal

17. Synthetic opal

18. Pearls
On the world market both natural and cultivated pearls can be found as well as their imitations.

Natural pearls:

• grow in bodies of molluscs without human interaction,
• after a foreign object has entered the body of the animal (parasite, larva, a fragment of a shell – not sand),
• often they have no nucleus,
• the layers alternate regularly from the core - organic conchiolin (4 – 14%) and aragonite (modification of CaCO3, 82–92 %),
• they are still very expensive.

Cultivated/cultured pearls:

• grow in molluscs bodies following human interaction,
• a foreign object is inserted (different material, tissue of a dead mollusc, etc.) into the animal´s body
• they can have a nucleus or can be without it (a tissue is inserted),
• they are sold marked as e.g. Seawater, Akoya (Japan), South sea pearls (Tahiti, Australia), Freshwater (China, USA),
• they are rather cheap,
• they are grown for example in river deltas in Asia in baskets hung under pontoons.

Pearl imitations:

• plastic matters, glass (different types, often with cavities inside),
• „Shell pearls“ –  from molluscs shells, coted with a coloured layer,
• they are very cheap.

19. Necklaces with cultivated pearls

20. Synthetic pearl imitations (were not cultured in a mollusc´s body)

21. To differentiate imitated pearls from the natural or cultured ones is very easy base on their surface morpohology.
Image from an electron microscope with a typical peal morphology, which grew in a mollusc´s body.

22. The problem is to differentiate natural pearls from those cultivated ones, however, the difference in price can be up to several orders of magnitude.. In such a case, it is necessary to carry out either a detailed examination of the chemical composition (e.g. isotopes) or examination of the internal structure (e.g. using an industrial micro CT).

Image of internal structure of a cultivated pearl using micro CT

23. The natural opal

24. Moissanit

• silicon carbide (SiC),
• crystallography - hexagonal, or even cubic,
• density: 3,218–3,22 gcm3,
• hardness: 9,5 (Mohs hardness scale),
• refractive index: nω=2,654 nε=2,967 (better than a diamond, that has 2,42),
• optical dispersion: 0,104 (2.5 times higher than diamond),
• used mainly clear for diamond imitation,
• first synthesis in 1884, introduced to the gemstone market in 1998.

vytisknout  e-mailem  X Corp.   Facebook