(also see: Crystal)
We almost always call these “gems”, for whatever reason
0x52AECC contains the number of gems you have collected in the current level (independent of save slot). As you would expect, it starts at 0 and goes up to 5.
There is another address which contains the same information, but with benefits we will get to later. However, it is not a static address and reaching it via pointers might not be possible (see here). The good news is, you can find it easily by searching memory for:
63 72 79 73 74 61 6C 00 47 47 45 66 66 65 63 74
(the strings “crystal” and “GGEffect”) while inside a level. The address is located 0x24 bytes back from the start of the strings (the red address in the screenshot below)
The number of gems is stored shifted over by 12 bits (which doesn’t seem to be uncommon in this game; see this comment from hdc0). So the screenshot above shows 2 gems collected (value of 0x2000; in the screenshot the least significant byte is on the left)
The advantage of this address over the more convenient 0x52AECC address is that you can set it to 4 gems (0x4000), then collect just one gem to conveniently spawn the Golden Gobbo for testing, without having to collect all 5 gems
Here is a convenient Lua script written by hdc0 that automates setting the value
(note that while you can simply freeze the value at 4 gems, this could have unintended effects when leaving/re-entering the level, because the address is not static…)