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.)
Nothing spells doom for your opponent like a blow from a marble baked in a volcano! This sphere began life in a volcano as red-hot molten lava. When this certain type of lava cools quickly, and under the right circumstances, it forms obsidian, also known as nature's glass. Some of these midnight black examples exhibit a shimmering effect in bright sunlight. Suitable for collecting or play.