I think your marble site is wonderful. I am so pleased to be able to find real rock crystal marbles. I use them in jewelry making. Finding 1/2" to 3/4" undrilled rock crystal beads has been nearly impossible. I'm glad I finally found a source.
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".