This is a lovely vintage stick or tie pin, which is set with a large faceted clear stone of approx. 0.65 carats.
The stone is 5 mm in diameter. It does not test as a diamond. The tester does, however, go up a fair bit, which shows it is a gemstone of some sort, possibly a white sapphire, cubic zirconia or similar.
The pin is solid 9 carat gold, which is hallmarked for Birmingham 1968. There is also a maker’s mark, which looks like “something” M. Co. (see photo). The claw top of the pin is a white metal, white gold or silver, which is typically used when setting diamonds and is another sign of quality.
The entire stick pin is 6cm long and is in excellent, undamaged condition.