Quartz Worktop Jointing Compound

Hi,

We have quartz worktops and the jointing compound that connects one unit to the other has chipped out between some joints (see attached picture). This stuff is rock hard and after a bit of research it seems like worktops like this are joined together with some sort of epoxy. Please could someone kindly point me towards the correct product to fill back in the bits that have chipped out? Also, is it a fairly straightforward job?

Thanks in advance