The formula for Mana Surge calculations:
SURGE CHANCE = (MASTERY*0.73) / (MASTERY+62)
This is called a "diminishing returns" formula. As you get more and more Mastery, the increase in Surge chance goes down. For example, 25 Mastery points give about a 20% Surge chance, but if we double that to 50 Mastery points, we now only see a Surge chance of a about 32%. In Gems of War, as you gain more Gem Mastery points the chances of a Mana surge dramatically slow down near the end.
Formulae like this are used in games to avoid imbalances for players at the end of the game.
An example of a game that uses a similar formula is League of Legends, with its damage reductions from Armor & Magic Resistance.