The definition of griefing

[partyars]

2) Griefing

Generally, “griefing” is anything you do that interrupts another player’s game experience. It can be as simple as ruining an RP experience or to destroying another player’s building for no valid reason. The extent of claimed griefing will be up for interpretation by the staff members.

2.1) Replace Any Removed Blocks

In the case of a robbery, where you are breaking in through anything that may be in your way, replace any blocks that you have removed. This also applies to when you are searching for hidden chests. The naturally formed world is not subject to the same idea. Not replacing blocks you destroy counts as lesser or greater griefing depending on the amount of blocks broken.

2.2) Exceptions

There are some blocks that are not considered building blocks and may be stolen at any time:
- Iron Block / Ore
- Gold Block / Ore
- Diamond Block / Ore
- Lapis Lazuli Block / Ore
- TNT
- Wheat (Please do replant it if you have the time)
- Melons, pumpkins (only if next to a stem)

If your character is planning to steal something, but you are not sure if it will be considered griefing, ask a Game Moderator about it. The moderators are there to help you with just this sort of thing!

Copypasta from the rules, seems pretty clear to me.