The Purāṇas list the nikkha (“ingot”) as the largest gold coin, but here a siṅgīnikkha must be much more than a hundred of the largest coins, and hence cannot be a coin at all. Siṅgīsuvaṇṇa means “mountain gold” (AN 3.70:38.3). Given that the next step is the entire earth, perhaps siṅgīnikkha means “golden mountain”. A “golden mountain” (pabbatassa suvaṇṇassa) appears in a similar context at SN 4.20:3.2.