Agate, chalcedonies

Chalcedony is the name given to a group of compact microcrystals of quartz in which crystals are so tiny that they are felt as a dense mass. Among quartzes there are fibrous and granulated kinds; the fibrous type meets more often and becomes monochrome stones or has strip- like or tree-like inclusions.

Chalcedony and agate are the precious stones. During long time the jewelers used this material as ornamental stones. Masters of "ar nuvo" style and the applied art opposed the colorful cabochons to the jewellery with brilliants. Work with chalcedony and agate demands the big skill; German stone-cutters from Idar-Obershtejn, having mastered this skill, create the real masterpieces.  

Chalcedony and agate estimation

Though there are a lot of these stones, it is not necessary to think, that chalcedony and agate are banal and inexpensive stones. High quality chrysoprase can be sold for high price, and some kinds of agate are extremely expensive .

The specification

Hardness

from 6,5 to 7 on Moos scale

Density

from 2,58 to 2,64

Factor of refraction

from 1,53 to 1,54

Crystal lattice

trigonal - mainly the fibrous units having the porous nature. Chalcedony and agate have a wax dim shine

Chalcedony kinds

Chalcedony is often found in a kind of transparent grayish or honey-yellow stones. This stone is porous and it can be painted. Natural chalcedony has an even colour without strips  

Dark blue chalcedony

This stone is very beautiful - it is painted in flower shades from gentle lilac to bright-blue colour, dark blue and smoky lavender. The best stones are transparent; low-grade chalcedony is a little bit foggy. The popularity of dark blue chalcedony has increased its price during the last years, and the stones with intensively-dark blue colour are rather expensive.              

Carnelian

For many years carnelian was a very popular stone, its warm shades involved both clients and jewelers. The colour of the stone varies from intensive orange-red colour to softer brownish-orange and red. High-quality stones are translucent and have a bright colour which sometimes strengthens by heating and coloring. Actually, these stones are agates colored and/or subjected to the heat treatment. Natural carnelian has a cloudy distribution of the colour, without strips and colour zoning. In old times the carved or engraved cameos were made of carnelian. Many Moslems respect this stone because consider that Prophet Mohammed carried it in a ring.  

Chrysoprase

The best chrysoprase should have a bright apple-green colour and to be translucent without distinctly expressed clouds. However, chrysoprase colour often varies depending on the inclusions of nickel in it and can be pale-green, emerald-green and dark green. The semi-transparency of the stone also varies. Colour is usually homogeneous and even, without zoning and stains. Inclusions in the form of black or brown stains can be observed; therefore the stone becomes similar to nephrite. Quite often to improve the colour chrysoprase is painted, but such stone costs cheaper. It is an ideal material for jewellers. Within many centuries it was used as the ornamental stone because of its attraction, durability and availability.  

Cornelian and sardonyx

Cornelian is painted in shades from reddish-brown to orange colour. Sardonyx is a striped cornelian. The strips have apricot and brown colour. This material is often used for a cutting of cameos and is excellent for incrustations.  

Onyx

In a natural condition onyx has direct strips of white and earthy-brown colours. Black onyx is not actually onyx but agate painted in black colour or chalcedony; it does not exist in nature. Agate was steeped in a solution of sugar and then got warm in sulfuric acid that parts of sugar have charred and have made a stone black. For many years the painted black onyx was used in jewellery.  

Bloodstone (heliotrope)

Opaque dark green stone with specks of red jasper. This stone is accessible and inexpensive. Traditionally it is used for man's rings.  

Chalcedony deposits

Chalcedony: The best materials arrive from Brazil and Uruguay, and also are found in Southern Africa, Indonesia and Turkey. Carnelian: India (a primary source), and also in Brazil, India, on Madagascar, to Russia (Urals Mountains), in Southern Africa, Tanzania, in the USA (California).  Cornelian and Onyx:  everywhere.  A heliotrope: India (a primary source), and also in Australia, Brazil, China, the USA.  

Kinds of agate

Agate arises in nodular masses which at disclosing show a surprising variety of remarkable colours and patterns.  

Tape agate

Have the strips of magnificent shades containing in the ring layers of a various thickness, and often contain white crystals of quartz. Both distinguish these stones from chalcedony. The colour of strips depends on impurities in them. Because of their porosity, these stones are often painted.  

Treelike landscape agate

Transparent pure white agate contains surprising "scenes" from black or dark brown treelike inclusions caused by oxides and hydroxides of iron. Black dendrites can look realistic, but actually their nature the same as in the ice crystals. Treelike agate is usually cut by slices and polished, then is inserted in pendants and brooches. The more attractively and detail image, the more expensively agate.  

Moss agate

Moss agate has received its name thanking to the moss black or brown inclusions of manganese or green inclusions of chlorides in translucent milky agate. Usually this material is cut by thin slices to show a pattern.   

Fiery agate

It is a surprising and unusual stone in which "oil" bubbles float on the dense-brown body of the stone. The fireworks of colours are caused by light diffraction in the oxides of iron layers in quartz. The accurate cutting shows the "game" of colours. The stone surface is often rough, as the stone-cutter cuts out some areas of the stone more deeply to put the colour layers more close to the surface.              

Agate deposits

Agate: agate mines in Germany (Idar-Obershtejn) now are exhausted, but there are huge deposits in Brazil and Uruguay. Agate can be found in Egypt, Italy, on Madagascar, in Mexico and Scotland. Moss agate: China, India, the USA.  Treelike agate: Brazil, India, the USA.  Fiery agate: Mexico, the USA (Arizona).