gold dust 
noun [mass noun]
1Fine particles of gold.He did offer patients who suffered from heart attacks an expensive cure that involved gold dust in a powder made from pearls, but he advised his colleagues to try to avoid hopeless cases as a matter of principle....
- Her maids had curled her hair and powdered it with gold dust, and were finishing an application of painted patches to her face.
- To portray the area she comes from, the dress will be black with gold overlay to mimic our history of miners searching for gold dust in black sand.
1.1British A thing that is difficult to find or obtain: council nursery places are like gold dust...
- Also avoid the Hungarian Grand Prix when available rooms are like gold dust and you'll need a bar of the stuff if you're lucky enough to find one.
- Our volunteers are like gold dust to Cancer Care we can't manage without them.
- Land's like gold dust in Eastbourne and there's plenty they could do with it in Swanley.
Synonyms
scarce, in great demand, at a premium, hard to come by, in short supply, thin on the ground, few and far between, not to be had, rare, rare/scarce as hen's teeth
informal not to be had for love or money
2A cultivated evergreen alyssum, with grey-green leaves and numerous small yellow flowers.
- Alyssum saxatile, family Cruciferae.