Is there a way to modify vanilla block's properties without coremods? I would like to lower Cobblestone from Stone drops and I can't find a way to modify Stone's properties.

 

If there was an API for 1.7.10 fulfilling this purpose, I'd also be happy with that.

Probably you will need ASM

Whenever you want to modify vanilla functionality, I find it is best to consider things in this order:

 

1) Look for public fields and methods in the class.  For example, entity AI list is public so you can totally change the AI of vanilla entities.

2) Use events.  Events are exactly for intercepting vanilla processing and there are events for most of the common stuff people like to mod.  I have a tutorial on events here: http://jabelarminecraft.blogspot.com/p/minecraft-forge-172-event-handling.html.  In your case, this is the way to go -- handle the HarvestBlockDrops event as suggested by diesieben07

3) Use Java reflection to access fields and methods that are not public.  Reflection isn't actually that difficult.

4) Consider replacing vanilla classes with ones that extend that class.  This can be a bit of work and possibly can affect compatibility with other mods, but sometimes makes sense.

5) Use ASM or other more advanced techniques for actual circumvention of vanilla code.