n. A small filament of gold that collects in the leaf of a tree that grows over a gold deposit.

Example Citations:
Here’s a new word for you: phytonugget. It’s a tiny bit of gold, the dimensions of which are roughly half the thickness of a human hair. It doesn’t sound particularly interesting until you hear that it grows on trees.
—Michael Brooks, “A market that won’t go pop: why helium balloons could one day cost £100 each,” New Statesman, November 7, 2013

Though the phytonuggets are too small to be collected and mined, they can serve as a sign that gold deposits may lie within the reach of a tree’s roots. Eucalyptus trees, which can grow lengthy taproots to reach deep ground water in arid areas, may stretch down 40 meters.
—Sid Perkins, “Gold in Trees May Hint at Buried Treasure,” Science, October 22, 2013

Earliest Citation:
Recent experimental studies on Au uptake by plants indicate that Au nuggets can form in the xylem tissue of plants. Although large concentrations of Au were used in these experiments, it does indicate the propensity for Au to form elemental particles, even in biota. Illuviated phytonuggets may be important as initiation sites for Au to nucleate in the soil or carbonate profile.
—Malcolm J. Lintern, “Calcrete Sampling — Twenty Years On” in CRC LEME Mineral Exploration Seminar - Abstracts, Cooperative Research Centre for Landscape Environments and Mineral Exploration, August 28, 2007

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