Beautiful in its simplicity, the luminous pure colour of chalcedony makes it a truly versatile stone to work with. Agate, on the other hand, provides an infinite number of patterns and textures that mesmerize, fascinate and are totally unique.

Chalcedony is the name given to a group of compact microcrystalline (or cryptocrystalline) quartzes in which the crystals are so tiny that they appear as a solid mass. The quartzes have fibrous and granular varieties; the fibrous type isthe most common and it appears as solid-coloured stones (such as chalcedony) or has banding or dendritic inclusions (as do the agates).

Chalcedony and agate are highly regarded stones, worn in many cultures as protection from the “evil eye”. They have also been used for many years as ornamental gems by makers of jewelry, such as the Art Nouveau and Arts and Crafts designers who employed these colourful cabochons as an antidote to the heavy diamond-set jewelry of the Victorians. Working with chalcedony and agate requires great skill; the German lapidaries of Idar-Oberstein excel at it and create true art forms in stone.

Treatments and imitations

  • Few imitations of agate exist. There have been attempts at reproducing the landscape “scenes” of dendritic agate using silver nitrate. Moss agate doublets have also been manufactured.
  • Buyers need to exercise caution when purchasing chalcedony and agate because of the amount of dyeing that goes on. Agate has been dyed since the 1820s at Idar-Oberstein, the centre for cutting and polishing agate. The dyeing process requires the use of special inorganic pigments, because organic colours fade in sunlight and are generally less intense. It is a complex procedure because the absorption of the dye depends on the degree of porosity of the different layers in the agate.
  • The colours of many dyed agates are so subtle that they are not obvious to the naked eye, so don’t assume a piece is natural just because there are no bright blues or greens. Although agate dyeing is seen as routine in the gemstone trade, the process should be disclosed to buyers. Natural-colour agates are more expensive, while low-grade, cheaper material is normally used for dyeing.
  • One indicator of dyed material is that agates kept in a plastic bag or plastic box will “sweat” in warm weather, leaving visible deposits or traces of the dye.

Pricing chalcedony and agate
Although the supply is plentiful, it would be wrong to think that chalcedony and agate are commonplace and inexpensive. Good-quality chrysoprase can sell for high prices and certain varieties of agate with special “scenes” (markings) are extremely valuable.

Working with chalcedony and agate

  • The colour of chrysoprase can fade when heated, so care needs to be taken during soldering and polishing. Colour can sometimes be restored by keeping the stones or rough material in damp conditions.
  • On the whole, agate and chalcedony are tough materials and resistant to chemicals. However, certain types of agate need to be treated with more care than others and should be protected from knocks. Jasper can separate along the layers within the stone, so banded or patterned material should be handled gently during cutting, polishing and setting.
  • The inclusions of dendritic agate lie at different depths within the stone, which is ground to bring them closer to the surface. As a result the stone can have an uneven surface form and some areas will be thinner than others. It is a good idea to check for any weak points before beginning to work with the stone.