What are Gemstones?

And
 How they are formed?


'Gemstone' is the name given to include all minerals, and some rocks and non-minerals, of beautiful appearance.

Rocks are minerals of atoms joined together, gold has only one group of atoms that are all the same kind, but most other minerals are a combination of different atoms.  They are not made from any animal or plant, like pearl, amber or coal.   They are a natural chemical.

 Minerals   and   Crystals 

 
  • Minerals have tiny particles called ions, atoms, and molecules. 
  •  Ions are groups of atoms that have an electric charge, molecules are groups of non-charged atoms. 
  • Minerals nearly always form crystals, each one has its own characteristic 3D shape.
  •  Most minerals contain oxygen.  Silicon is the next most abundant element.
  • Some minerals are not durable enough for general wear as jewelry, but are sometimes used for occasional wear and ornaments.  An example of this is feldspars from igneous rocks. 
  • A crystal is a mineral with several faces to it in its natural state in the ground. 
  • There are seven main ways a crystal can grow 
  • Billions of ions are linked together to form each crystal.  
  • In a crystal the atoms join together in an orderly way.  Not in a random messy way.
  • Some crystals are too small to be seen with the naked eye. 
  • To identify the crystal a device called a goniometer may be used which measures the angles between the face of a crystal.  Also the colour of a crystal will help identify it, but some crystals have several colours like quartz for example.


What is a non mineral?  

A number of natural substances like amber, jet and pearl.  They do not qualify as minerals because of their lack of definite crystalline structures.  What they are made of can have different arrangements, and they are made from organic (once living) material.

Amber is a natural fossil tree resin.   Pearls are natural secretions of carbonate minerals by oysters and other shellfish.  Jet is a material representing the early stage in the conversion of vegetable matter into coal.

 

Why are gemstones valuable?

Around 90 minerals are classed as gems, but only about 20 of these are important gems.

The different colours of gemstones are caused by metal impurities in the mineral.  So really the gemstone becomes valuable when some foreign metal is mixed into the mineral.  

For a gemstone to be valuable it has to be able to last, so it must have a hardness in its properties between 7-10 on the Moh's hardness scale of minerals. If it is softer and can be scratched,  it is lower than 7, then it will suffer a lot of wear and tear from sand for example.   See below for the Moh's scale.  Gold is a soft mineral, it can be bent, stretched, melted and shaped.  So metals are added to give gold strength. 

The rarer the gem is the more valuable it becomes.

 

Moh's scale of hardness for Gems and Minerals:

Hardness

Gem

Uses

10

Diamond

  

9

Corundum (Ruby and Sapphire)

8

Topaz

7

Emerald 

Quartz

  

Tool steel

6

Orthoclase

Opal

  

Knife / Glass

5

Apatite

Window Glass

4

Fluorite

  

Copper coin

3

Calcite

  

Fingernail

2.5

2

Pure Gold

Gypsum

1

Graphite

Talc

 

Each mineral will scratch the one below it on the scale.  Because of their hardness and/or resistance to chemical attack, many precious minerals survive long after the rocks in which they formed have been completely weathered and eroded away.  This is when you could be lucky in finding a gem just sitting on the ground.  They may also have swept along river beds in mud and sand, or formed in river valleys like gold is found in Australia. 

Go here for more information on the hardness of minerals. http://www.mcli.dist.maricopa.edu/aaim/linear/L1.html 

 

How are gems formed?

The original sources of many gem minerals are from ancient igneous rocks. Often found in granite where the molten rock crystallizes quartz, feldspar and mica. Veins often form in the rocks and over time the rock weathers and erodes and leaves behind the gem minerals.  

Gem cutters fashion and cut the stones to make the light reflect the gemstones best colours.

Gemstones are weighed as carats. A carat is 200 milligrams, or 1/5 (0.200) of a gram.

 

 

Pegamite is an example of an igneous rock like granite?

The pink mineral seen in this rock is Tourmaline which is often used as a gemstone.  To see more about this rock go to our Pegamite page.

 

Amethyst is one of the most popular gems that comes from the quartz rock which is an igneous rock.

See our page on Quartz to learn more about this gem and even more on our Amethyst page...

 

Gems also occur in sedimentary and metamorphic rocks.  Although crystals forming in sedimentary rocks do not have good crystal shapes.

 
This is Garnet Gneiss a metamorphic rock found in Western Australia. Garnets form in a wide variety of rocks in different parts of the world.  Some are formed from great pressure deep in the Earth others are formed in clay rich rocks that have been metamorphosed, and others are often found in granite rocks.  Even limestone, a sedimentary rock, that is altered by heat produces a type of garnet.   See our page on Garnet Gneiss to learn more about this rock. 

 

Metamorphism causes new minerals to develop in old rocks.  The large crystals of garnet in a schist rock were not in the original rock.  The type of minerals that grow depends on what the original rock was made of, and the strength of the metamorphism i.e. how strong the heat and pressure was on the rock. 

Some minerals are useful like asbestos and sulpher, while others such as onyx are just for decoration.  Some like diamonds and gold are used for both e.g. drill bits can be made from diamonds and heat is conducted by gold.

 
This is the yellow sulpher, it smells like rotten eggs!  Sulpher is not a gemstone.

Photographed at the Geological Museum Perth

 

 

DIAMONDS: 

The word diamond comes from a Greek word adamas which means "hardest metal". 
 Diamonds are also the most valuable of all gemstones.  They also reflect the light with the most intensity out of all natural colourless gems.     

See our page on Diamonds in this section to learn more about them.                

 

 

RUBIES and SAPPHIRES: Are both rare forms of the mineral corundum, which is very hard. 
 Small grains of corundum are called emery and are used on nailfiles.

See our page on Rubies and Sapphires for more information.

This corundum in the picture above is from Western Australia and is a metamorphic rock.  

 

Biotite schist of Western Australia.  

EMERALDS: 

Are found in mica schist from a metamorphic rock.   
Emeralds are a variety of minerals called beryl.  Emeralds usually have 50 facets cut in them. 
 See our page on Emeralds in this section for more information.

Because of their rarity, diamonds, ruby's and sapphires and emeralds are considered the 'true gems' and others are called 'semi-precious stones'.

 

OPALS: 

Photographed at the Geological Museum Perth

Opal forms by the slow deposition of silica from fluids, like water, passing along veins and cracks in a variety of sedimentary rocks such as sandstone, limestone and shale. Opals are cut into an oval shape and are multi coloured. See our page on opals.

 

Not all gems may be what they seem!  Did you know that gems can be made in a laboratory or factory?  They are called synthetic gemstones.  They are materials made up with the same chemicals as some natural minerals, but have been grown as crystals in a laboratory and not through time in the ground.  Sometimes a gem may be a natural gem but it has had colour added to it to make it look nicer.  Gems are made in a laboratory because people want to be able to wear jewelry that is not as expensive to buy as the natural gemstones.  

Some words that mean the gemstone is not a natural gem are; synthetic, created, and fake.  So how can you know if you have the 'real thing'? It takes an experienced person and sophisticated equipment to know the difference, but genuine gemstones are usually colder to touch than glass or plastic imitations.  And if you look closely under a magnifying glass you may see gas bubbles, a sure sign of a fake. 

See our page on "Gold" in the gems section to learn more about this precious metal..

 

Text used;

"Fashioning the Stone" by Alex Bevan and  "Rocks and Minerals" by Jack Challoner, and all these sites listed throughout this section on gemstones. 

Assignments:  Take a look at these pages   http://www.bsu.edu/teachers/academy/gems/activities.html and http://www.minerant.org/education.html We are not suggesting that you buy things over the net but you may gain some good ideas and guidelines for activities here, especially for home schooling.  Remember activities can be tailored to all ages. 

There are many good sites about gemstones.  These are just a few of them. 

http://www-wf.minerals.csiro.au/ScienceandMinerals.html In this site you will find these other sites listed below.  They are just a sample of what you will find here at this site.  Check it Out! 

 

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