Archaic KJV Word
Pearl
Modern equivalent: jewelry
What Was Lost
The total transaction. The merchant did not add the pearl to his collection -- he sold everything else to get it. The kingdom of God was not an addition to a balanced life but the one thing worth liquidating everything else to obtain. The pearl demanded total, exclusive, all-consuming commitment.
Closest Survivor in Modern English
pearl of great price (still used in Christian contexts, but broader culture has lost the all-or-nothing dimension)
Peak Usage (1611)
KJV Matthew 13:45-46 -- 'A merchant man, seeking goodly pearls: who, when he had found one pearl of great price, went and sold all that he had'
Died still used but jewelry-ized (~1800)
Greek margarites ('pearl/the most precious thing obtainable/what you would sell everything to possess') reduced to jewelry and fashion accessory. The all-or-nothing, sell-everything valuation was lost.
What Replaced It
“gem”
Decorative stone; the pearl of great price demanded total divestiture -- selling everything else
“jewel”
Ornament; the pearl represented something so valuable that rational people would liquidate their entire life to obtain it
“precious thing”
Vague; the pearl was specifically the one thing worth everything else combined