I was very impressed with your website. Found exactly what I needed to replace missing marbles for a game that I have. Also, I found some fun stuff for my grandchildren to play with while looking at this site.
Size is as-packaged, or the diameter of loose marbles. ("D" is the diameter of round items other than marbles.)
These genuine crystal quartz balls are exactly like the real thing, only smaller! They make dazzling marbles - every one has different reflections and sparkle inside. These are great for collecting too. 18mm is about 23/32".