"But this way it looks more like an acorn.... What's that? 'A windfall, unexpected gold.' Excellent, you can lend me some..."
-- Ron Weasley, consulting Unfogging the Future while reading tea leaves (PA6)
An acorn is the fruit of an oak tree, and something many animals and birds put away for the winter. Perhaps that's why it's associated with hidden gold.
In European folklore, according to D. C. Watts in the Dictionary of Plant Lore, a dream of an acorn foretells wealth and health (2). -BB