Chalcedony Species Part 4 Agate

Hi everyone,

Welcome back to the next installment in the series that is Chalcedony.  We have covered many different types of chalcedony in the previous series.  Today we will be discussing just Agate.  There are many types of agate and we did cover some in previous posts.  Today I would like to go more in depth with agate.  As in previous posts all the information that I used to write this blog is from the book Gemstones of the World by Walter Schumann.  So let's talk agate!

The colours of agate are all colours banded.  The colour streak is white and the Mohs hardness is 6.5-7.  The density is 2.60-2.64 and there is no cleavage.  The crystal system is trigonal microcrystalline and the chemical composition is silicon dioxide.  The transparency is translucent to opaque.

agate with great banding

Agate is a form of chalcedony that has concentric shell-like bands which may sometimes contain opal.  These bands can be multicoloured or all of the same colour.  The name is thought to have derived from the river Achates which is now called Drillo in Sicily.

The origins of agate are usually ball or almond shaped nodules and have a wide range of sizes.  They can be anywhere from a fraction of an inch to having a circumference of several yards.  It is thought that rhythmic crystallization is how the bands are formed.  There are many theories as to how agate is formed.  The old theory was that the bands were formed by crystallizing slowly in hollows that were formed by gas bubbles from a siliceous solution.  The more recent theory is that the bands are formed simultaneously with the matrix rock.  What this means is that when the  liquid drops of silicic acid cool with the cooling rock this produces a layered crystallization from the outside.  There is a new theory that postulates that colloid solutions (substances with very fine sized grains) flow into the agate hollows and forms the bands of various thickness.  There are times when the inner cavity is not filled with agate but with crystals.  Some examples of this are rock crystal, amethyst and smoky quartz.  When only the centre is filled it is called a druse but if the inside is completely filled then it is called a geode.

Nice selection of agates

There are many types of agates.  There is the eye agate so named because the bands form a ring shaped design with a point in the centre similar to an eye.  Layer agates have layers or bands of similar size parallel to the outer wall.  Dendric agate was already covered in previous post.  Enhydritic agate (water stone) is a mono-coloured chalcedony partly filled with water which can be seen through the outer walls.  Fortification agate is when the bands have corners that jut out like bastions of old fortresses.  Fire agate is an opaque limonite-baring chalcedony with an iridescence that is created through diffraction of light.  Orbicular agate has circles of agate layers, either arranged concentrically or excentrically around a centre point.  Moss agate and scenic agate have been discussed in a previous post.  Tubular agate has numerous tubes or old feeding canals.  The thunder egg of sandstone is a layered agate with strongly furrowed outer surface.  Brecciated agate is broken agate cemented together with quartz.

Agates can be found in Germany which were very important deposits in the 19th Century but these deposits are mostly gone now.  The agates in Germany could not be dyed as they came in beautiful colours of grey, pink, red, yellow, brown and pale blue naturally.  The most important deposits today come from Brazil and Uruguay.  Other deposits can be found in Australia, China, India, Caucasus, Madagascar, Mexico, Mongolia, Namibia, and the United States.

I hope everyone enjoyed learning about agates today.  I had fun writing about it today and I learned alot as well.  Please let me know how I am doing.  I would love to hear from you.  Drop me a line.  Until next time.

The rare gem lady

Chalcedony Species Part 3 - Dendritic Agate & Moss Agate

Hi everyone,

Welcome back to the next in our series of Chalcedony.  Today I would like to discuss dendritic agate and moss agate.  These stones I find very interesting in their colour and the way one can use these stones to create very interesting works of art.  The way that these stones are cut and polished makes them look like ferns or landscapes.  The colour are rich and intense.  I think that these stones are very unusual and unique.  I hope you enjoy learning about these stones and that you have fun as well.  As before all the information being used in this blog is from the book Gemstones of the World by Walter Schumann.

Dendritic Agate

Dendritic agate is usually colourless or a whitish-grey colour.  The chalcedony is translucent usually with tree or fern-like markings.  The name is derived from the Greek dendrites meaning tree-like.  There are some people who feel the term agate is not correct as this stone has no bandings, however must people do not consider the term to be misleading.  The dendrites are iron or manganese inclusions and are usually brown or black in colour.  This look is formed in very fine fracture surfaces through the crystallization of weathered solutions in the neighbouring rock.  The dendritic chalcedony are found with other chalcedonies.  The deposits are normally found in Brazil, India, and the United States.  There are scenic agate which is a dendritic agate in which the dendrites resembles landscape-like images in brown or reddish colour tones.  Then there is the mosquito stone.  This is where the dendritic agate has ball-like growths which is reminiscent of swarms of mosquitoes.  These types of stones are used for rings, brooches, and pendants.

Scenic Agate

Mosquito Agate



Moss Agate



Moss agate is a colourless chalcedony with green, brown or red inclusions.  The colour streak is white and the Mohs hardness is 6.5-7.  The density is 2.58-2.64 and there is no cleavage.  The crystal system is trigonal microcrystalline and the chemical composition is silicone dioxide.  Moss agate is translucent and has inclusions of green hornblende or chlorite in a moss-like pattern, hence the name.  The moss agate that has the red and brown colours are from the oxidation of iron hornblende.  As above the name agate is generally accepted even though there is no banding.  The appearance of the stone occurs through filler in fissures or as pebbles.  These rare gems are found in India, China, Russia and Colorado.  Most often used as plates, cabochons for rings, brooches, and pendants.



Moss Agate Rough



I hope everyone enjoyed this journey of discovery into the world of dendritic agate and moss agate.  I had fun learning about the new information discovered.  I would like to hear from you.  Please let me know what you think about this post or any others that I have done.  Until next time.

The rare gem lady

Cryptocrystalline - Chrysoprase and Bloodstone

Hi everyone,

Today I would like to discuss the next part of the cryptocrystalline family and talk about chrysoprase and bloodstone.  These two rare gemstones are very unique and different.  I like the look of bloodstone.  The unique patterns that are in them.  I hope everyone will enjoy the learning and the journey we are going to take on the path to chrysoprase and bloodstone. Also just a note that all the information that is being used in this blog is from the book Gemstones of The World  by Walter Schumann.

So let's talk chrysoprase.  The colour of the gem is green, apple-green and the colour streak is white.  The Mohs hardness is 6.5-7 and the density is 2.58-2.64.  There is no cleavage and the chemical composition is silicon dioxide.  The transparency is translucent to opaque.  According to Mr. Schumann the gemstone chrysoprase is considered to be the most valuable of all the gemstones in the chalcedony group.  The name is Greek meaning gold-leek.  The fine quartz fibres have a radial structure.  The colouring agent is nickel.  The colour can fade in the sunlight and when heated but may be recovered under moist storage.  The gemstone occurs as nodules or filling in clefts in serpentine rocks and in weathered materials of nickel ore deposits.  The stone can be found in Australia, Brazil, India, Kazakhstan, Madagascar, Russia, Zimbabwe, South Africa, Tanzania and California.  Chrysoprase is used as cabochons, necklaces and ornamental objects.  In earlier centuries is was used as a decorative stone for the Wenceslaus Chapel in Prague, and in the Sanssouci Castle in Potsdam.

Chrysoprase stones

The bloodstone is a dark-green chalcedony with red spots and is an opaque stone.  A name that is still used in Europe is Heliotrope which is Greek for sun turner.  The green colour is caused by particles of chlorite or included hornblende needles.  The red spots are caused by iron oxide and the colours are not always constant.  According to Mr. Schumann the most important deposits are found in India but are also found in Australia, Brazil, China, and the United States.  The bloodstone is often used as seals on men's rings and for ornamental objects.  There is a trade name that is sometimes used which is blood jasper but that can be confusing as bloodstone is not a jasper.



bloodstone gemstone

 I hope that everyone had a great time learning and journeying on the path to chrysoprase and bloodstone.  I had fun and I learned some really interesting things today.  I hope you will drop me a line and let me know how I am doing.  Until next time.

The rare gem lady

Cryptocrystalline Quartz - Chalcedony Part 1

Hi everyone,

I would like to discuss the cryptocrystalline quartz family today.  Again this will be a multi-part series as there is a lot of information to be shared.  I hope you will join me on this journey of discovery and learning.  I think this is an interesting section of the gemstone family.  In this area are some unique stones and I really like to learn about new things.  Today we will be talking about Chalcedony, Carnelian, and Sard.  I would also like to state that all the information being provided in today's blog is from the book Gemstones of the World by Walter Schumann.



Blue Chalcedony



So let's talk chalcedony.  In his book, Walter Schumann states "Chalcedony is used by gemologists as a species name for all cryptocrystalline quartzs(eg: agate, petrified wood, chrysoprase, bloodstone, jasper, carnelian, moss agate, onyx, and sard) as well as specifically only the bluish-white-grey variety, the actual chalcedony."  Chalcedony has the appearance of being dull or waxy in its natural state.  The description of chalcedony given now is in the narrow definition of the bluish-white-grey chalcedony.  Colour is bluish, white and grey and the colour streak is white.  The Moh's hardness is 6.5-7 and the density is 2.58-2.64.  There is no cleavage and the crystal system is trigonal, fibrous aggregates.  The chemical composition is silicon dioxide.  The transparency is dull, translucent.  The name chalcedony is from the ancient town at the Bosporus.  The stone itself, consists of microscopic fibers,which are parallel to each other.  The stone shows macroscopically radiating, stalactitic, grape-like or kidney shapes and it is always porous, which means the stone can be dyed.  Natural chalcedony has no banding.  This stone can be found in Brazil, India, Madagascar, Malawi, Namibia, Zimbabwe, Sri Lanka, Uruguay and California.  There is also Chrome Chalcedony which is the trade name for the natural green colour of the stone due to chromium found in Zimbabwe.



Chrome Chalcedony



Carnelian is most likely named after the colour of the kornel cherry due to its colour.  The stone is a brownish-red to orange.  It is a translucent to opaque member of the chalcedony family.  The colouring agent of the stone is iron and its colour can be enhanced by heating.  This type is found in Brazil, India, and Uruguay.  When held to the light natural carnelian has cloudy distribution of colour.  Sard is a red-brown to brown variety of the chalcedony family.  There is no real strict separation from carnelian.  The stones that tend to be darker or more brown are called sard.

Sard

I hope everyone had fun today and learned something new.  I know I had fun and learned new things.  Please let me know how I am doing.  Drop me a line, I like to hear from like minded individuals.  Until next time.



Carnelian



The rare gem lady

Quartz Species Part 5

Hi everyone,

Today I would like to finish up the macrocrystalline family.  I would like to say that it has been very interesting to learn all the neat things about quartz so far.  I have enjoyed the journey so far and I hope you have too.  Today I am going to discuss Quartz cat's-eye, Hawk's eye and Tiger's eye.  Very earthy and animalistic today!  Please join me on my journey and I hope you have fun and learn new and exciting things today.  So let's being the learning!  As before I am using the information found in the book Gemstones of the World by Walter Schumann as the basis for all the information I am using in this blog.

So let's talk Cat's eye quartz.   The colour for cat's eye quartz is white, grey, green, yellow, and brown.  The colour streak is white and the Mohs hardness is 7.  The density is 2.58-2.64 and there is no cleavage.  The Crystal system is trigonal, usually massive and the crystal composition is silicon dioxide.  The transparency is semi-transparent to translucent.  The quartz cat's eye has numerous fiber-like inclusions of rutile create an effect called chatoyancy.  Chatoyancy is the description for the play of light on the surface which shows the effect of  cat or bird or animal of prey. This type of quartz can be sensitive to some acids.  These quartz can be found in Sri Lanka, Brazil and India.  When the stones are cut as a cabochon it shows the chatoyancy like a cat's eye which is cause for the name.  When someone calls a stone cat's eye without the quartz name attached to it most people then take the stone to mean the chrysoberyl  version of the cat's eye.

Hawk's eye is finely fibrous opaque aggregate that is formed when quartz replaces the mineral crocidolite.  The colours range from blue-grey to blue-green and iridescence of planes and fractures have a silky luster.  This type of quartz is also sensitive to some acids.  This stone is used primarily for ornamental objects and costume jewellery.  Cabochons are cut to show the chatoyancy, which is reminiscent of the bird of prey's eye.

Tiger's eye is formed from Hawk's eye where the iron from the decomposed crocidolite has oxidized to a brown colour.  The luster is silky on the fractures.  This type of quartz shows chatoyancy stripes, because the structural fractures are crooked or bent.  Tiger's eye quartz is usually found in South America, Australia, Burma, India, Namibia and the United States.  This type of quartz is used for necklaces, costume jewellery and objets d'art.  The cat's eye appearance happens when the cabochon is cut to show that effect.

I hope everyone had fun today.  I really enjoy learning new things and I hope you do too.  Please let me know what you think of this or any other posted blog.  I would love to hear back from you so please drop me a line to let me know what you think.  Until next time.

The rare gem lady

Quartz Species Part 4

Hi everyone,
I would like to continue the conversation we are having about the quartz family.  This is part four of a 5 part series of the macrocrystaline quartz family.  We will then take a look at the cryptocrystaline quartz family after that.  I hope everyone is having fun and learning new things along the way.  My goal is to have fun and educate people on the types of gems out there.  I have such a passion for learning about jewellery and gemstones.  In today's blog we will be discussing Rose Quartz, Aventurine, Prase and Blue Quartz.



star rose quartz gemstone



So let's talk rose quartz first.  As I have stated before all the information that I am using for this blog today is from the book Gemstones of the World by Walter Schumann.  The colour range of rose quartz is strong pink to pale pink and the colour streak is white.  The Mohs hardness is 7 and the density is 2.65.  There is no cleavage and the crystal system is trigonal, prisms mostly compact.  The chemical composition is silicon dioxide and the transparency is semitransparent to translucent.  Rose quartz is so named because of its pink colour often has a crackled appearance.  The colouring agent is titanium.  If there are rutile needles this causes a six-rayed star appearance when cut as a cabochon.  Rose quartz is found in Brazil, Madagascar, India, Mozambique, Namibia, Sri Lanka, and the United States.



Next on the list is Aventurine.  The colours for adventurine vary from green, red-brown, gold-brown and the colour streak is white.  The Mohs hardness is 7 and the density is 2.64-2.69.  There is no cleavage and the crystal system is trigonal, massive.  The chemical composition is silicon dioxide and the transparency is semitransparent to translucent.  The name of adventurine comes from Italian a ventura.  In around the year 1700 a type of glass was discovered and the same name was given to the stone that looked similar to the glass.  The look of adventurine is mostly a dark green with metallic glittering appearance which is caused by fuchsite(green mica) or red to gold brown which is cause by hematite leaves.  Aventurine is found in Brazil, India, Austria, Russia, and Tanzania. Aventurine is primarily used for ornamental objects and cabochons.

Prase is a leek green quartz who's name is from the Greek word prason.  It is usually classified as chalcedony and its colour is caused by chlorite inclusions.  Prase is usually found in Saxony, Germany, Finland, Austria, and Scotland.

Blue quartz is a turbid-blue quartz aggregate also known as quartzite.  The colour of blue quartz is caused by inclusions of crocidolite fibres.  This stone is usually found in Brazil, Austria, Scandinavia, South Africa and Virginia.  Primarily used for ornaments.

I hope everyone had a good time learning about the pink and blue quartz, the aventurine and prase today.  Please let me know what you are thinking about this series or about any of the other blogs written.  I would love to hear from you so please drop me a line.  Until next time.

The rare gem lady

Quartz Species Part 3 Citrine and Prasiolite

Hi everyone,

Here we are again on our journey through the wonderful world of the quartz family.  I am learning so much from the books and websites that I have looked at and visited.  I find that the quartz family or species is very interesting as there are so many colours and varieties to this family.  I hope that you will learn something new and interesting in this blog today.  I find that I always do.  I hope you have fun.

So let's talk citrine and Prasiolite.  I would like to start with citrine first.  I also would like to mention that the information that I used to write this blog has come from the book Gemstones of the World by Walter Schumann.  Citrine has the colour range of light yellow to dark yellow, gold brown and the colour streak is white.  The Mohs hardness is 7 and the density is 2.65.  There is no cleavage and the crystal system is hexagonal, trigonal.  The chemical composition is silicon dioxide and the transparency is transparent.

Citrines name is derived from the lemon yellow colour of the gemstone.  The colour agent for citrine is iron.  According to Mr. Schumann natural citrines are very rare.  Most of the citrines found on the market today are heat-treated amethysts or smoky quartz.  Brazilian amethysts are heated at 878 degrees F to produce a light yellow colour.  To achieve a dark yellow or red-brown colour the amethyst has to be heated to 1022-1040 degrees F.  To make a smoky quartz a citrine colour the gemstone must be heated to 390 degrees F.  To tell if a citrine has been heated treated look at the colour.  If it has a reddish tint to it, it has been treated.  If the gemstone is light yellow in colour than it is most likely a natural citrine.  There used to be names that were used to describe citrine but are no longer acceptable.  These names are Bahia, Madeira and Rio Grande topaz.  The only acceptable use is when they use it to describe the colour.  An example of this would be Madeira Citrine colour.  Citrines are usually found in Brazil, Madagascar, the United States, Argentina, Burma, Namibia, Russia, Scotland and Spain.

Prasiolite's colour is leek green and the colour streak is white.  The Mohs hardness is 7 and the density 2.65.  There is no cleavage and the crystal system is hexagonal, trigonal.  The chemical composition is silicon dioxide and the transparency is transparent.  The name prasiolite is Greek for leek green stone.  It is not a naturally found gemstone.  Prasiolite is created by heating violet amethyst or yellowish quartz from the deposit Montezuma in Minas Gerais, Brazil to a temperature of around 930 degrees F.

I hope that everyone that reads this blog today has had a fun time and learned something new in regards to citrine and prasiolite.  I enjoy writing about new gemstones and learning about new gemstones.  Please drop me a line and let me know how I am doing.  I would love to hear from you out there.  Until next time.

The rare gem lady