Fluorite is calcium fluoride, and it might be the most colourful mineral on Earth. It comes in purple, green, blue, yellow, pink and often several colours banded in a single stone. The famous Blue John from Derbyshire in England is a banded purple fluorite mined since the 1700s. Fluorite also gave its name to fluorescence, because many pieces glow blue-violet under UV light. Major sources include China, Mexico, South Africa and England, and it often grows in perfect glassy cubes.
Here is the trade-off: fluorite is soft, only 4 on the Mohs scale, and it has perfect cleavage in four directions, so it chips and scratches easily. It is really a stone for pendants, earrings and display rather than rings. Faceted fluorite is spectacular but strictly for gentle wear. Keep it away from ultrasonic cleaners, heat and knocks, and store it separately. Purple fluorite is sometimes passed off as amethyst; the giveaway is that a steel pin will scratch fluorite but not quartz.
Purple fluorite is a classic amethyst look-alike, though a lot softer.
