I have to say, Ducky’s purported lapse of character in this episode wasn’t as extreme as all the hype had led me to expect. I think the reason her behavior comes off as so atypical is because so few movie and episode plots focus on Ducky that viewersóby no fault of their ownóhave a rather narrow and stereotyped view of her character. Therefore, virtually anything Ducky does that does not fit this paradigm is regarded as out of character for her. (It might help if the series had a greater number of Ducky-centric LBT stories; I would REALLY like to see more attention given to this surprisingly neglected character.)
There
is some prerequisite for Ducky’s behavior in this episode: she has shown possessiveness before, complaining when Spike ate the “special tree stars” her mother had given her (presumably for lunch) in LBT VIII. Going from jealously valuing personal effects to secretly hoarding them isn’t too great a step.
I think the trouble in this episode started when Cera and Ruby started arguing over which of them could find the most stones. Apart from swimming and log-running, poor Ducky is at a disadvantage in just about every competitive activity her friends engage in. In this contest, however, the odds of winning were more or less equal, and with Ruby and Cera having turned up the competitive atmosphere, Ducky was motivated to prove that this was something she could do just as well as her friends. And while I may be going out on a limb here, the fact that Ducky is usually an inconsequential opponent in most of the gang’s competitions means that her larger friends might not have taken her seriously had she openly expressed interest in joining the hunt for the most sky color stones; perhaps this was another reason she decided to participate so surreptitiously.
Ducky’s initial desire to simply prove herself as a competent rock-finder probably evolved into her obsession with simply taking all the stones for herself when she started carrying the sky color stone Spike found. Its aesthetically captivating appearance caused her to become possessively attached to it, and as a result she became reluctant to share it or any of the other stones she found. (I’m a rock and mineral enthusiast myself, and unusual and/or remarkable stones have a similarly mesmerizing effect on me.

)
The problem I DO have with the episode is the way it portrayed Ducky’s descent into kleptomania and jealous guarding of the stones: at the best it didn’t feel believable to me, and at the worst, downright melodramatic. Of course, cheesy moments (unfortunately) appear to be something of a staple of the TV series, so I suppose this isn’t too surprising.

On a side note, if I ever had the chance to take Ducky along with me on a rock-collecting trip, I’d probably do it. She obviously has a good eye for rocks (identifying a “cave sparkle” as well has finding all those sky color stones), and being much closer to the ground than I am, is far better suited for the task of scoping out desirable specimens (I’m six feet tall, nearsighted, and have bad knees; pretty much the worst combination imaginable for walking along looking for tiny objects on the ground

).