Where do stones get their color and why do some fade?

So I came across this question on Etsy and tried to give a very basic explanation why some minerals are colored the way they are, and why some will fade when exposed to light/heat. I thought this might be something others would be interested in and thought I'd share what I wrote here :)


Color and fading has to do with the composition of the mineral. Basically what happens is when some minerals are exposed to sunlight/heat, the electrons get rearranged, resulting in color loss. This only occurs in some minerals, because there are different ways minerals can get their color.

Idiochromatic minerals have their color due to their chemical composition, such as malachite and azurite. Thus their color is pretty constant. Malachite for instance is always green - there may be varying shades of green, but never any other color but green. Were you to take away the component that results in the green color, you would simply no longer have Malachite, but some other mineral.

Malachite is always a shade of green.


Allochromatic minerals get their colors from trace impurities in their composition or defects in their structure. Fluorite and Quartz are examples. Therefore they're prone to color change - again it also varies from mineral to mineral as some are more resistant to others depending on composition and structure. Unlike with Malachite, were a trace impurity removed that caused a certain color (e.g. remove the iron and aluminum responsible for the purple color of Amethyst, which is a type of Quartz), you still have the same mineral (i.e. while the stone would no longer be purple, it would still be Quartz).

Amethyst is a type of Quartz that is purple due to iron and aluminum impurities.


Here's an article I found that includes a brief discussion on color loss in Fluorite - its a reprint from Rock and Gem magazine http://members.aol.com/Florspar/articleF1.html

And this one explains why minerals have color and a bit about color variety occuring in the same mineral http://geology.csupomona.edu/alert/mineral/color.htm


When I have time I'll try to find some more info on this :)

In the meantime, I recommend always storing jewelry out of direct light and away from sources of heat, so you can continue to enjoy the beautiful colors of your stones for years to come!

Stone of the Week - Pyrite

Pyrite is a metallic mineral that is comprised of iron and sulfur with the chemical formula FeS2. It belongs to a group of minerals called “sulfide minerals” and is the most common mineral of the group. It can often be found with other sulfide minerals (as well as other mineral types), often in quartz veins, coal, sedimentary or metamorphic rocks. The name pyrite comes from the Green word “pyros”, meaning fire, due to the fact that pyrite can spark if struck. It looks so much like gold to the untrained eye, that it has also earned the nickname “fools gold”. In fact sometimes gold can be found in the same locations as pyrite, making it even harder to tell which is which! However they can be told apart, as pyrite is lighter than gold, yet pyrite can’t be scratched with a fingernail or pocket knife like gold can because it is harder.

Pyrite is a metallic mineral that is a bright, brass yellow color, and found in a variety of forms.


Pyrite can be found in a variety of forms, from massive accumulations to various crystal forms. The most recognizable is the cubic form, which often have striations running across the surface. It has a hardness of about 6–6.5, actually making it softer than quartz; additionally it can also be brittle and break in an uneven or somewhat conchoidal manner. However pyrite is heavier than many common minerals, with a specific gravity of almost 5. When pyrite is exposed, it can react with both oxygen and water, resulting in the formation of acid.


Pyrite can be found in massive forms, sometimes readily seen along surfaces of rocks, such as this rock outcrop of metamorphic gneiss located in Canada.


There are several minerals that are very similar to pyrite. Marcasite is what is called a ‘polymorph’ of pyrite, meaning that while it has the same chemical formula, it is structurally different, and thus a different mineral. Chalcopyrite has copper added to it (CuFeS2), and is also a brassy yellow color, but is softer and lighter than pyrite (can you think of why this might be?). Bornite has even more copper and added sulfur (Cu5FeS4); you may know this mineral as “peacock ore” as when it tarnishes it turns from a copper-red to iridescent blues and purples.


Peacock ore is a technically called Bornite, and has a similar chemical composition to Pyrite. When it oxidizes these amazing colors can form as a result!


Pyrite has been used in the past for use in fire arms due to its ability to spark when struck. In more modern times it has been used to produce sulfur dioxide or sulfuric acid for industrial use, though this is not as common as it used to be. Pyrite in recent years has become available for use as beads or pendants for jewelry work. Care must be taken when using it – be sure to NEVER store it completely sealed without something to absorb moisture. Many a collector has stored pyrite in air tight containers or zippy bags, only to find a slushy mess a few months later. This is because the pyrite will react with the moisture and air trapped with it, resulting in the disintegration of it and the production of acid in its place (hence if you come across this situation handle with care!)

Rarely, fossils can undergo a process called "replacement", where the fossil is replaced with a mineral (while keeping its original form); in this case an ammonite found in Russia has been replaced with pyrite.



If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!


Pyrite Facts:
Chemical composition: FeS2
Crystal System: Isometric
Color: Pale brass-yellow
Habit: Massive, Crystals (often cubic)
Fracture: Uneven, Slightly Conchoidal
Cleavage: Poor
Luster: Metallic
Transparency: Opaque
Hardness: 6–6.5
Specific Gravity: ~5g/cm3
Streak: Greenish-black
Occurrence: Worldwide



References

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Pyrite. http://en.wikipedia.org/wiki/pyrite

Mindat – Pyrite. http://www.mindat.org/min-3314.html

Stone of the Week - Sodalite

Sodalite derives its name from the sodium content of this mineral; the chemistry of this mineral is Na4Al3(SiO4)3Cl, or Sodium Aluminum Silicate Chloride. It belongs to a group of minerals called feldspathoids. Feldspathoid minerals have poor silica content, and in the case of sodalite, it has absolutely no silica (aka quartz) at all. In fact, this mineral doesn’t even form if there is quartz around – it would react with the quartz to form another mineral. Since quartz is so abundant on Earth, there is sodalite is not a common mineral.

Sodalite often appears as a mixture of colors, and rarely found in a solid blue mass.


Many are familiar with the royal blue color of sodalite, however it occurs in a variety of colors including shades of blue, color less, white, light yellow, green, gray, pink, and violet. Hence it can sometimes be difficult to identify sodalite from other minerals. In its commonly sold blue form, one way to distinguish it is its streak. This the color the powder of the mineral leaves when ran across a ‘streak plate’ which is commonly a piece of white porcelain, however in a pinch you can try doing this on a white piece of paper. *Be sure to do this using a ‘fresh’ surface of the stone – trying this with polished surfaces such as what you get with beads doesn’t work – you need to expose a fresh surface of the stone.


The blue/blue+white variety is most often used for creating beads and decoration.


Sodalite is only found in quantity in a few select locations – mainly in Canada, and a few states in the USA, though a few smaller deposits can be found on other continents. There are a few different types of sodalite, one of the more interesting being Hackmanite. This particular form of sodalite has a property called “tenebrescence” which results in the stone changing color when exposed to light! (This same property is used in industry to create those glasses that will darken when exposed to sunlight!)


This mineral can make for some very pretty and unique jewelry!


Sodalite is a beautiful stone today used mainly for jewelry work, and sometimes decoration and sculpture. One story has it that in 1901, the Prince and Princess of Wales (the future King George V and Queen Mary) were presented with a piece of sodalite. The princess loved the beauty of this stone so much that she had tons of it shipped back to England where she used it to decorate the Marlborough house. Years later this mine was renamed the “Princess Sodalite Mine” in her honor.


Sodalite Facts:
Chemical composition: Na4Al3(SiO4)3Cl
Crystal System: Isometric
Color: blue, color less, white, light yellow, green, gray, pink, and violet
Habit: Massive, rarely crystals
Fracture: Uneven, Conchoidal
Cleavage: Dodecahedral (six directions), poor
Luster: Vitreous, Greasy
Transparency: Crystals are transparent to translucent, massive specimens are opaque
Hardness: 5.5-6
Specific Gravity: 2.27-2.33
Streak: White
Occurrence: Worldwide, but most commonly in Canada and the US
Other: Fluorescence



References

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia - Sodalite. http://en.wikipedia.org/wiki/Sodalite

Wikipedia - Tenebrescence. http://en.wikipedia.org/wiki/Tenebrescence

Mineral Galleries - Sodalite. http://mineral.galleries.com/minerals/silicate/sodalite/sodalite.htm

Geological Garden gets 4-ton rock. http://newsrelease.uwaterloo.ca/news.php?id=1004

Stone of the Week - Celestine or Celestite

This is one of my favorite minerals and I love to collect it so much, hardly a gem show passes that I don’t buy a new specimen of it each year! The official name of this mineral is celestine, however it is still commonly called celestite, and is named from the Greek word "cœlestis", meaning for celestial due to its pretty pale blue color. Celestine is made up of strontium sulfate, or SrSO4, and is similar enough to another mineral, barite (BaSO4), that these are sometimes misidentified. This mineral commonly forms in sedimentary environments.

The pretty blue celestine crystals are a favorite among collectors.


Celestine comes in a variety of colors, including colorless, white, yellow, orange, gray, green, brown, and pale blue. It is the pale blue that is most commonly sold both as specimens and as beads. The introduction of celestine in the jewelry market is fairly recent, and can make for some beautiful jewelry. However, care must be taken as it is a soft stone, and can more easily be scratched than harder minerals such as quartz. It is also very slightly soluble (dissolves) in water, so do not expose it to water or moisture for any extended period of time. It is also best not to expose it to the Sun for extended periods if you do not wish the colors to fade.


These blue celestine crystals occur with white aragonite and a few purple fluorite crystals.


The worlds largest geode, located at Crystal Cave, Ohio (refer to previous article on geodes), is made of celestine. It is 35 feet at its widest point, and has crystals as wide as 18 inches across, estimated to weigh 300 lbs. Also of interesting note, there is a micro-organism called Acantharea (radiolarian protozoans) which have skeletons of strontium sulfate (aka Celestine). Celestine has sometimes been mined as a source of strontium, but always in very small quantities.


Crystal Cave, the worlds largest geode, is made up of large celestine crystals.



If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!


Celestine Facts:
Chemical composition: SrSO4
Color: colorless, white, yellow, orange, gray, green, brown, pale blue
Habit: prismatic and tabular, also massive and granular
Fracture: uneven
Cleavage: prismatic and basal
Luster: vitreous
Hardness: 3-3.5
Specific Gravity: 3.9
Streak: White
Occurrence: Worldwide, mainly in the northern US, Europe, and Madagascar


References

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Mindat – Celestine. http://www.mindat.org/min-927.html

Wikipedia – Celestine. http://en.wikipedia.org/wiki/Celestine_(mineral)

Wikipedia – Crystal Cave. http://en.wikipedia.org/wiki/Crystal_Cave_%28Ohio%29

Stone of the Week - Copper

Copper is an elemental metal, represented as Cu on the periodic table. It is a very ductile metal, making it easy to shape and work with. Copper does not react with water (H2O), but does react to the oxygen in the air, forming a layer of brown-black copper oxide on copper metal. The green layer of copper carbonate (sometimes called a patina) that can form actually slows the corroding process. One of the most famous examples of this process is the green patina that has formed on the Statue of Liberty in the U.S. Copper can be found in mineral form as native copper, as well as in minerals such as chalcopyrite, azurite, malachite, and cuprite. Copper is very malleable as well as ductile, conducts heat well, and if near pure conducts electricity too.



Copper is mined and has a vareity of uses, particularly as jewelry components.



Due to the properties of Copper, it has a variety of uses, including copper pipes, copper wire, copper pans, and electronics. A special property of Copper is that it also can be used as an antibacterial and antifungal agent! Copper kills germs due to the toxic effect the metal ions has on bacteria, fungus, and mold spores. For this reason Copper has become more integrated particularly with hospital equipment. One example of this is brass doorknobs, which become disinfected within about eight hours or so.



This is what Copper looks like in a more 'raw' form as nuggets. Notice the green patina and the beginnings of oxidation occuring on the nugget to the left.



Copper exists in various locations through out the world, and has been used since ancient times. It has been in use for over 10,000 years, with the oldest known Copper pendant dating to 8700 BCE. Numerous artifacts comprised of Copper have been found world-wide, mainly as the alloy Bronze. Several copper alloys have been developed for different uses; common alloys with copper include bronze (copper and tin) and brass (copper and zinc). Today the world's largest mine for Copper is located at the Bingham Canyon Mine, located in Utah (U.S.), and is actually so large it is viewable from space!

Copper is a great alternative metal to use in jewelry work and is useful for practicing techniques as it is cheaper than Sterling Silver or Gold filled metal.

Copper has made a popular entrance into jewerly work recently as well. Although a slight greenish tint can occur when wearing Copper, numerous people have no reaction to it or don't wear the jewelry long enough for this to be a concern. Should this occur it can be easily washed off; additionally, some also try to coat the metal to protect both it and the person wearing it such as clear nail polish.

If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!


Copper Facts:
Chemical composition: Cu, plus various alloy compounds
Color: Brown to coppery red
Habit: Branching and dendritic, can be massive
Fracture: Rough
Cleavage: None
Luster: Metallic
Hardness: 2.5-3
Specific Gravity: 8.9
Streak: Red-brown
Occurrence: Worldwide, some of the largest deposits in N. America and the UK

References
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Copper. http://en.wikipedia.org/wiki/Copper

Stone of the Week - Garnet

Garnets are actually a group of minerals, of which there are 15 different mineral species. Each mineral has a similar chemical make up -> X3 Z2 (SiO4)3 where X and Z can be different elements. It is these different combinations that make up the variation of minerals, although Calcium (Ca), Magnesium (Mg), Aluminum (Al), and Iron (Fe) are the most frequent. Out of the different minerals, there are several common garnets: pyrope (dark red), almandine (red to violet red), spessartite (yellow, rose, orange, reddish brown), grossular (white, yellow, yellow-green, brownish-red, orange, black), and andradite (colorless, yellow-green, brown, black). Within each of these are several varieties as well - hessonite and tsavorite are both types of grossular garnets for example.

This display at the National Museum of Natural History in the US shows some of the variety of color just within the grossular species.


Some of the most valuable garnets are also the rarest. Blue garnets are a type of pyrope garnet, that was discovered in the late 1990s in Bekily, Madagascar, and has since been found in the US, Russia and Turkey. It is one of the few garnet species that changes color from blue-green in the daylight to purple in incandescent light. Another sought after garnet is Tsavorite, which belongs to the grossular garnets, and is a brilliant green color. Tsavorite garnets were first discovered in a deposit in Tanzania during the 1960s, which extended into Kenya. The only other known location for these rare garnets is in Madagascar. Uvarovite is another bright green garnet that is one of the less common species, and is found in Russian and Finland.


Uvarovite is a spectacular green garnet, and is often sought after for jewelry.

Garnets are common world wide, particularly in metamorphic rocks such as marble and schist; pyrope garnets are usually associated with igneous rocks. They are abundant in the US, Brazil, South Africa, England, and Australia. Gemstone quality crystals are used in jewelry work, the most common being the red garnets. It is also the birthstone for January. Garnets have also been used as an abrasive for sand blasting, in cutting, and as part of some water filtration units.

Garnets can be found in nice crystal forms, such as the grossular garnets on the left. However the deep reds are a popular color found in jewelry.

If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!



Garnet Facts:
Chemical composition: X3 Z2 (SiO4)3
Crystal System: Isometric/Cubic
Color: Varied – red, green, yellow, brown, black, blue, orange, clear
Habit: Well formed crystals are common, as well as massive and granular.
Fracture: Conchoidal
Cleavage: Indistinct
Luster: Vitreous, Resinous
Hardness: 6.5-7.5
Specific Gravity: 3.4-4.6
Streak: Colorless/White
Occurrence: Worldwide


References
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Garnet. http://en.wikipedia.org/wiki/Garnet

Stone of the Week - Fluorite

Fluorite, also known as Fluorspar, is made up of calcium fluoride (CaF2). The name fluorite is derived from the Latin fluo, meaning "flow", because it is slightly dissolvable in water. Fluorite belongs to the Isometric crystal system, otherwise known as the cubic crystal system. This means that its most basic form (properly called a ‘unit cell’) is a cubic shape; hence the crystals are commonly cubic.

Fluorite can be found in a vareity of beautiful colors including blue, pink, blue-green, and purple.

Fluorite can occur as a variety of colors, including green, blue, purple, pink, yellow, and white. These colors can occur as angular banding, commonly referred to as ‘chevron’ banding. A neat property of fluorite is that it is fluorescent – in fact this is where the term fluorescent came from. When held under UV light, the electrons inside the fluorite become ‘excited’ resulting in their energy level increasing – when they come back down from this ‘excited state’ light is released. Usually in fluorite this will be blue, but other colors are known to occur.

Fluorite can often be found as cubic crystals (left) and a bit more rarely this gorgeous chevron banding (right).

In the United States, the state of Illinois is the most abundant location for fluorite, and is the official mineral of Illinois. This area is often called the ‘fluorspar district’, and was mined until 1995 when the last mine closed – now it’s a popular location for rockhounds to collect specimens. Another famous locale for fluorite is in Castleton in Derbyshire, England. Here there is a unique blue and purple fluorite called ‘Blue John’ and is used for ornamental stone. Fluorite is a popular stone for jewelry work, though care must be taken as it is a ‘soft’ stone with a hardness of 4 on the Mohs hardness scale.

Fluorite makes for a beautiful stone to use in jewelry and ornamental work.

If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!


Fluorite Facts:
Chemical composition: CaF2
Crystal System: Isometric
Color: blue, green, purple, pink, white/clear, yellow, brown, red, black
Habit: Crystals, Granular, Massive
Fracture: Uneven, Conchoidal
Cleavage: Perfect
Luster: Vitreous
Transparency: Transparent
Hardness: 4
Specific Gravity: 3.3
Streak: White
Occurrence: Worldwide, but most commonly in the US and UK
Other: Fluorescence, some specimens may exhibit thermoluminescence

References

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Fluorite. http://en.wikipedia.org/wiki/Fluorite