"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)
Tea leaf symbol from Unfogging the Future meaning “windfall, unexpected gold.”
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.