About pearls and nacre (mother of pearls)

A pearl is a gemstone produced by mussels when some irritant get into the flesh near to the shells. The irritants most potent to stimulate the mussel to produce a pearl are organic particles and parasites, not sandgrains as usually thought. The mussel produce the pearl around the ittitant to catch it and shield it from itself. This reaction from the moussels is probably a protective reaction against parasites.

Many species of saltwater and freshwater mussels can produce perls, and so can also some snails. The freshwater pearl mussel (Margaritifera margaritifera) is just one example.

The substance at the inside of a mussel shell is the same as that substance perls are made of, called mother of perls or nacre. This substance consists of very thin concentric layers of of miroscopic calcium carbonate chrystals in the form of the minerals aragonite and chalcite glued together with a horn-like substance called concholine, which is a blending of organic substances like chitine, lustrine and silk-like proteins.

Because the crystals in each layer are not aligned directly over each other, the light is diffracted when going from one layer to another and some of the light is mirrored back at the top of each layer. This way the pearl gets a diffuse shiny luster and a diffuse play of colors called irrdiscence. The thinner the layers are, the finer will the structure of the irridiscence get. Pearls with especially fine layers that are nearly perfectly round are regarded as the most beautiful and are generally higher prized.

Most perls have a greyish basic color, but some pearls are very dark, called black pearls.

Natural pearls of good quality are very rare. Therefore pearls are produced by cultivation of mussels and by introducing some irritant into the flesh of the mussels, for example peanut butter.

Commercially available pearls can be of several origines: Saltwater natural pearls, saltwater cultured pearls and freshwater cultured pearls. Some pearls originate from big saltwater snails.

Nacre (mother of pearls) is the inner layer of the shells of muosles and of the houses of snails. It is composed of layars of the calcious minerals aragonite and chalcite with layers of choncholine between.

Nacre shows an irridiscence of the same type as pearls and pieces or plates of nacre are used as decorative elements on many objects or as gemstones.



How do gemstones get their colors and appearance

Most gemstones consist of one or more crystals with atoms or molecules (consisting of several atoms) bound together in a regular framwork fashion, called a crystal lattice. Most gemstones, at least in their final shape, consist of just one crystal, fro example diamonds, rock crystals, rubys amd emeralds.

Some genmstones consit of myriads of microscopic crystals, this kind of structure is called microcrystalline, for example agath and onyx. Also perls and mother-of-perls are microcystalline. The same is true about noble materials like  ivory and other beautiful animal teeth.

Some gemstones ar not crystallic at all, but some kind of glass, for example obsidian (volcanic glass) and opal.

And one type of gemstone, namely ember, is actually a sort of natural hard plastic. It is formed from resin from trees that have been chemically and physically harded through thousands or millions of years.

The physical composition is very determining for how the gemstone will look. Monocrustalline gemstones are often limpid and transparent. They are also able to refract the light and thereby give an raibowish appearance. The greater the ability of refraction is, the more vivid is the raibowish play of light.

Microcrystalline gemstones can be translucent, but never transparent. The refraction in all those crystals can give specific fascinating effects, but not like a plain rainbow. If for example the crystals are orderd into thin layers over or adjasant to each other, the effect can be like the beautiful shiny effect in mother-of-perl where the colors gradually shift from place to place and shift gradually also when you change the angle of sight. If the layers are thicker and  have different colors, the gemstone can show beautiful bands of different colors.

Gemstones with compsition like glasses or plastics are often transaparant or nearly transparant.  But also these can have a layered structure that shows an apparance like mother-of-perl.

White light consists of light with a lot of wavelengths, perceived as different colours when they hit the eyes. When white light falls into the atoms in the crystal lattice, some of the wavelengths are absorbed by the individual atoms or by the structure of the lattice. The energy in the waves absorbed is converted to heat.

Some of the light that is not absorbed is reflected from the surface, and some will be let through the gemstone.

Since the light reflected and the light comming through will have selected wavelengths, this  light will look coloured.

However, tho light falling on a gemstone is not allways perfectly white. If one of the wavelengths normally let through or reflected lacks from the light falling in, the gemstone will look different than in white light. If the light falling in totally lacks the wavelengths normally let through, the gemstone will look black in that light.

The crystal lattice can have foreign atoms or small foreign molecules dissolved or trapped between the atoms making up the framework of the crystal, for example iron, copper, cromium or water. Also these dissolved elements can absorb light of certain wavelengths and contribute to the colour of the gemstone.

Different pieces of the same basic type of stone can have different dissolved substances. Many types of gemstones therefore are found in different colours, and even one and the same stone can have different dissolved substances from place to place, giving the stone colour shadings. Sometimes different coloured gemstones of the same type has different names. An example of this is corrundum that can be found in the gemstone variants rubuine and saphire.

Some types of atoms in a stone often give some typical colours. Iron will often give a red or brown colour, copper will often give the stone a green colour, cobalt gives blue.

It is often possible to change the colour of a stone by dissolving foreign elements in the stone. Agats are often artificially coloured in this way.

Light falling into a gemstone will be refracted, but the different wavelengths will be refracted in different degrees. Thus the different wavelengths comming through will be spatially separated and the stone will show several simultaneous colours. Therefore the refraction process will also contribute to the colour of the stone. However, colours from the refration process will vary according to the angles from which one regards the stone, and the angles from which the light falls into the stone.

Gemstones can be composed of many crystals composed together, or be cut in a complicated way with many facets. A crystal can also have a lot of tiny cracks in the lattice. In these cases the stone will show a very complicated colour play resulting from simultaneous refraction in all parts of the stone. Diamonds are usually cut in such a way to maximize this process.

Certain atoms or crystal structures absorbe light, but sends the energy immediately out again in the form of light with a different wavelength. This is called fluorscence. Flourescence is also a way by which a stone gets its colour.

In some rare instances, the energy absorbed is stored in the stone for some time, and sent out again slowly in the form of light with a characteristic colour. A gemstone having this property will seem to glow in the dark. This phenomenon is called phosphorescence. Ultraviolet light from the sun can sometimes cause phosphorescence in some minerals.

Sometimes the crystal of a stone contains inclusions of foreign elements that are greater than dissolved individual atoms or molecules. The size of such inclusions can vary from microscopic objects to small grains visible by the eye. They can be small crystals of other minerals, small drops of water or small drops of oily substances. Such inclusions can absorb wavelengths of their own, they will refract light and can show fluroescence. Such inclusions will give a gemstone a complicated colour play. An example of this is the gemstone opal that contain small drops of water giving the stone a complicated colour play.

If the inclusions are great enough, they can also give the gemstone a myriad of tiny spots of different color than the ground color of the stone.


What substances or structures make the different colours

Colors can originate form the main components or the crystal lattice, but often the colors comes from impurities in the crystal lattice, replacing the regular ions.

Mostly colors originate in metal ions form the transient groups in the periodic table. These have an incomplete set of 3d-electrons. The elecrons in these elements can absorbe visible light of certail wavelengths, and thus light passing through or being reflected will lack these wavelengths and thus become colored.

In other instances color originate from impurities of atoms having another number of valences that the main atoms of the lattice. This makes it possible for binings to be broken and new bindings reappear with another valence electron. There may be an energy difference between two such althernative bindings and this shift can be made by absorbtion of energy from frequences of visible light. Such inclusions are called color centers.

In still other gemstones there is exchange of electrons between two adjacent transition metal ions of differing oxidation states. The energy needed to transfer an electron from one ion to another corresponds to the energy of visible light. This is called intervalence transition.

Clear / colorless - In this case little light is absobed, and the light absorbed are of all wavelengths. Crystals having cilicone or calcium as positive ions / elements in the lattice, generally are white. Examples are quarts crystals.

White - Also in this case little light is absorbed, but there are millions of small elements whithin the stone or crystal that defract and reflect the light.

Red - Red often originate in cromium present as impurities in the shape of cr3+-ions. Red may also come from iron (fe2+) present as impurities or as apart of th ecrystal lattice itself.

Green - Green is also often caused by cromium (Cr2+) in the lattice, and sometimes iron (fe2+)

Blue or bluish-green - Copper (Cu2+) often makes this color.

The color caused by a ion cannot allways be predicted, because adjacent ions also play a role. The following table shows how different elements can cause different colors in various minerals:

Alexandrite	Al2BeO4	Red/Green	Cr3+ replacing Al3+ in octahedral site
Diamond	C	Colorless, pale blue or yellow	Color centers from nitrogen  atoms trapped in crystal
Emerald	Be3Al2(SiO3)6	Green	Cr3+ replacing Al3+ in octahedral site
Garnet	Mg3Al2(SiO4)3	Red	Fe2+ replacing Mg2+ in 8-coordinate site
Peridot	Mg2SiO4	Yellow-green	Fe2+ replacing Mg2+ in 6-coordinate site
Ruby	Al2O3	Red	Cr3+ replacing Al3+ in    octahedral sites
Sapphire	Al2O3	Blue	Intervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
Tourmaline	Na3Li3Al6(BO3)3(SiO3)6F4	Pink	Mn2+ replacing Li+ and Al3+ in octahedral site
Turquoise	Al6(PO4)4(OH)8 • 4H2O	Blue-green	Cu2+ coordinated to 4 OH¯ and 2 H2O
Aquamarine	Be3Al2(SiO3)6	Blue	Intervalence transition between Fe2+ and Fe3+ replacing Al3+ in adjacent octahedral sites