Amethyst is the purple variety of the quartz mineral species. It’s the gem that’s most commonly associated with the color purple, even though there are other purple gems such as sapphire and tanzanite. … Amethyst’s purple color can range from a light lilac to a deep, intense royal purple, and from brownish to vivid. Source : GIA.Edu
First of all if you have any of your old ring, you can keep it on a straight table, and you can use any scale on that ring, and count that mm.