If the game crashes, or you have to stop playing and play later, there is no "normal" way to save your progress, forcing you to play through the entire campaign in one sitting, without a break.
I have found a solution, although it can also be used as a "cheat" to unlock all the gems if you are not able to get them all and want to see the secret ending.
METHOD:

You need admin access to console or sv_cheats 1 activated.
When playing any map, write on console the required lines for every gem (1 to 5, and red gem) you want to be recognized as "collected":

global_set Gem1Collected 1
global_set Gem2Collected 1
global_set Gem3Collected 1
global_set Gem4Collected 1
global_set Gem5Collected 1
global_set RedGemCollected 1

The gems collected will be stored on memory and on the next round restart or the next map, will be visible on the start room automatically.
If you do this on the last map, kill yourself (and all human players) to reset the map or the secret ending will not activate.