the gem will actually tel you its cap it will say 0/50 ranks meaning you have 0 of the 50 ranks possible.
if you rank gems for the caldesan enchant always use gems that dont have any cap at all just to be safe.
and never ever use the gem of ease for caldesan it is the only gem with a special use in the cube it can be used to remove the lvl requirement of an item (at rank 25) so a new lvl 1 character can use a lvl70 item.