I don't receive much physical mail these days, it's mostly just advertising bumph that goes straight into the recycling bin. That almost happened to the brown envelope too except the address's spidery handwriting sent a shiver down my spine. I'd seen that handwriting before.
Opening the envelope revealed this tattered old newspaper cutting:

At the bottom, in that same spidery hand, was written "Want to find it?" Nah, as a keen geocacher I'm not into finding stuff, especially not long lost diamonds worth a king's ransom. Of course I want to find it!
Digging into the envelope again, I found an equally tattered map similar to the one I was given at a funeral a few years back and published as GC9M6X5.

Like last time, the only thing labelled is the Middle Sea, which turned out to be Brisbane Water, and the shape of the shoreline looks much like an area I explored years ago when doing some old Geomonkeys caches that have since been archived. Going back through my notes, I found lots of photos I'd taken there, including ones of an ancient redgum and an outcrop of rocks that look much like the illustrations on the map. Again like last time, I even have coordinates for them!
So being a caring and sharing geocacher (or just no good at multis), it's the same deal again. Clicking on the map will open a larger version on which you might see some extra pointers to help in your search. The listed coordinates will take you to that grand old fire-ravaged tree where you'll find a log bearing the first clue to the diamond's whereabouts. Stage 2 is the outcrop of rocks where, if you search a ledge in the cave underneath, you'll find a flat rock bearing the second clue. The diamond is nearby at:
S 33° 29.??? E 151° 18.???
After that, you're on your own, except to say that some rock scrambling might be needed, with easier access from west of the cache. Good luck in your quest, you'll need it!
CAUTION: The rocks around GZ get very slippery when wet.
Congratulations to crash2079, the first to find the diamond!