Blue John
Typical of the north south divide, if in Derbyshire, called Derbyshire Fell Spar, if in Cornwall called Blue John.
Basically white quartz, a semi precious stone, made by copper and metal solution leaching through rock into the quartz making a vein of blue and brown colour.
Large pieces are rare, most mines are exhausted, when you next go to a country house with the National Trust, you will often see 2 or 3 large items such as goblets or pots and covers, the larger and finer colours the more sought after.
Blue John should not be confused with amethyst as the later is all purple where as Blue John has the trade mark zig-zag purple/blue lines and stripes.
A by product of copper and metal mining which became very popular for use as a statement often of wealth as as got rarer was added to jewellery. Whilst this one pictured unusually is gold, most is mounted in silver and the metal lends itself very well to enhancing the stones.
Semi precious, is not a measure of worth its a measure of hardness, most jewellery stones are semi precious, diamonds, sapphire, ruby, emerald and aquamarine are the 5 precious stones, due to the pressure they have been placed under the ground making them super hard.
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