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That's funny, because now I don't know where I got it from. Anyway, once I learned (from John original post above) that isolating the saturation was a workable solution to this very difficult selection, I have fine-tuned the approach:

First, I noticed that the beads are yellow-ish. So in order to even further distinguish the saturation of the beads vs. the background, I opened a Hue/Saturation adjustment and greatly increased the saturation of the Yellow channel.

[ATTACH=full]136976[/ATTACH]

Then I added the Selective Color adjustment layer, using the technique I described above:

[ATTACH=full]136977[/ATTACH]

Then I added a curves adjustment at extreme settings to boost the blacks and whites. There's a little manual cleanup required, but this final image would make a very good mask.

[ATTACH=full]136978[/ATTACH]