Many months ago, John once explained how to isolate the saturation component of an image, so I wrote it down in a Word file that I keep that contains instructions for various techniques. Here's how to do it:
Above the image, open a Selective Color adjustment layer. At the bottom, click the button for "Absolute".
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Now go into the drop-down for each color channel. For each of the colors Red through Magenta, set the Black slider all the way to the left at negative 100%.
For the last three color channels (White, Neutrals, Blacks), set the Black slider all the way to the right to positive 100%.
In the resulting image, the darker an area is, the less saturated it is. The brighter it is, the more saturated.
In this particular situation with the beads, John then added a curves adjustment to boost the contrast.
