Peridot is a beautiful green gem that is the birthstone for August babies. It is a green stone that is either loved or hated for its yellow green colouring. Quite often in gems when a green stone is discussed people think of Emerald or even Jade but Peridot has a charm all its own.
The colour of peridot is part of its chemical makeup-a term that is called idiochromatic or self colouring. Iron is the chemical in the formula of Mg2Fe2SiO4 that gives colour to this gem. Other gems may get their colour from a chemical that changes in the formula and thus gives more colours such as in Corrundum. As a gemmologist, when I think about Peridot I think about the Fosterite (Mg2SiO4) -Faylite (Fe2SiO4) series of stones. These stones form an Isomorphous Replacement series where one element replaces another element in the formula and although the form and crystal structure remains the same the physical properties may change. As you can see Peridot is a combination of both Chemical formula thus placing it in the middle (roughly) of this series.
Most times we see Peridot as a yellow-green gem but it can also be green or even a brownish green. However the brown green is quite rare. An interesting property of Peridot is that when you look at the back or pavillion facets of a peridot through the table you may see doubling of the facet edges. This phenomenon is called doubling and to a gemmologist it would indicate a Double refractive stone. This is a term for a light ray when entering the stone splitting into 2 rays and travelling at different speeds. We measure this phenomenon (and all gemstones have their own ) as a Refractive Index. In the case for Peridot the RI is 1.65- 1.69 hence being able to see the doubling.
Another fun but distinctive part of peridot is one of the inclusions that may be found inside a peridot. It is caused by a chromite crystal during the growth of peridot causing stress cracks within the crystal structure and looks like a lily pad. How cool is that?
So now you know a bit more about peridot and why it is special enough to be my birthstone-the birthstone for August, the month of Leo.
Enjoy your month and when next you see a peridot take a closer look-you never know what secrets it might share with you.