There are more than 79 agates. I am currently at 85 agates without the 10 agates from the crimson wishes. (Tree Level 9.6)
Also on the topic of whether this one is hard to miss or not, this is definitely a very easy one to miss as it does not show up on the mini map(inside chest). I myselfs confused this island with one of the more inland ones and passed by it thinking I had already been there.
So before you go bragging to OP about it being hard to miss, It would probably be a good idea to find the ones your missing first.
As for OP, thanks for pointing this out. If you find any more that you have missed feel free to keep pointing them out, you never know who might be missing one.