This is green fluorite. It is another natural unenhanced gemstone not widely known. This gems has a bluish green colour as opposed to a bright green. It has a very slight change in tone of colour depending on the light. In daylight /sunlight it is a brighter green and in lower lighting it has a darker hue.
Either way it is pretty and very inexpensive.
This specific piece is almost flawless as you can see. You can see clearly into the centre of the gemstone . This is a nice size to wear on the neckline as it is over 10cts . I set this into sterling silver and the chain will be included.