The list is up-to-date as of April 14th, 2016.
One time rewards.
|Without limited content: 5735|
|With limited content: 8208|
|Total : 1665|
|Vasco da Gama||120|
|Fernao de Magalhaes||256|
|Isabella of Castile||155|
|Samuel de Champlain||130|
|Total : 570|
Some of the things above do not award gems directly, but award materials which can be traded for gems. The above list assumes that all of those are traded for gems.
Repeatable/Recurrent ways of getting gems
Whenever a regular Shipwreck appears on the map, there is a 30% chance it will be a special one, which contains materials that can be traded for gems on the market.