The salt of the earth
Those of great worth and reliability.
The phrase 'the salt of the earth' derives from the Bible, Matthew 5:13 (King James Version):
Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men.
The positivity towards salt in this phrase conflicts with many other uses of the word salt, which has also been used express negative concepts; for example, in the Middle Ages, salt was spread on land to poison it, as a punishment to landowners who had transgressed against society in some way.
It seems that the 'excellent' meaning in 'the salt of the earth' was coined in reference to the value of salt. This is reflected in other old phrases too, for example, the aristocratic and powerful of the earth were 'above the salt' and valued workers were 'worth their salt'.
'The salt of the earth' was first published in English in Chaucer's Summoner's Tale, circa 1386, although Chaucer undoubtedly took his lead from Latin versions of the Bible:
Ye been the salt of the erthe and the savour.