GregTech 5 ore blocks are not normal blocks. Because of the enormous range of ores GregTech can generate, all GregTech ore blocks are created as non-ticking TileEntities. However, for bandwidth reasons, only ore blocks that are visible to players (i.e. not completely surrounded by other blocks) have their TileEntity data sent to them. This has a few odd side-effects. First, if a player is using an X-Ray mod or hack, while the ore blocks will appear, they will be blank and unidentifiable. Second, if any other mod attempts to detect ore blocks in the area (such as the Rod of the Plentiful Mantle), they would not detect them as ore blocks. And third and most commonly, in cases of server lag or network bugs, GregTech ore blocks can temporarily appear as featureless gt.blockores.0 blocks. This can also happen in cases of common server lag, when the client breaks an ore block but the server does not register it immediately and places the block back again, before actually breaking and dropping it a few seconds later.
gt.blockores.0 blocks do not drop anything when broken, and as such users should not attempt to break them.
Under most circumstances, these blocks should revert to actual ore blocks almost immediately. If they do not, log out then log in to the server again (forcing the identifying packets to be re-sent). If problems persist, consider filing a bug report.