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Gold was always the ultimate form of money in the material world. Gold, silver, copper - there was no other money back then. So, it is.

"The eye of a needle" is scripture quoting Jesus recorded in the synoptic gospels:

I tell you the truth, it is hard for a rich man to enter the kingdom of heaven. Again I tell you, it is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God.

Matthew 19:24