[Massena]

From Sunray's magic site database, if all sites are equally probable then:
Level 1 mage finds 40-50% of gems in a province
Level 2 mage finds 70-80%
Level 3 mage finds 90-95%
Level 4 mage/Spell finds 100%

So...a Rainbow mage (level 2 in 7 types of magic) will find 5 times (75%x7) more gems than a specific spell - they won't necessarily be all as useful though!