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