As with several Caithness weights, "Mooncrystal" was issued in two styles. The first was in 1985 and was produced in "many colour variations". The later style was in 1997 and as at 2004 was an active unlimited edition. Della was nearly correct in the number of colours - the catalogue book states 6 (Black, Blue, Green, Hyacinth, Pink and Yellow).
Both the earlier and later ones were in Medium (62 mm) and Miniature (54 mm) sizes. [The mm sizes seem not to be given in the 2nd edition of the book, so the ones stated are from the 1st edition.]
From the photo of Paul's weight, I guess that it's a 1997 or later one. But the photos in the book are only b&w and it's not always easy to tell the difference in design styles - as is the case for this one, but the earlier one seems to have a bubble half way up with no higher swirly bits!