Hi Petamarie, welcome aboard, I'm assuming fossicking for you means not detecting but in the creeks looking for colours.
Have a good read under the title of Gemstones, Minerals and Fossils on this forum.
Gumeracha, Birdwood, Williamstown and stretching around to Palmer that area in general can spring surprizes on fossickers, farmers in the Williamstown area have reported finding gemstones in the discards when digging bores, unfortunately those gravel beds lay some 10 metres underground but it is a clue to some of the geological events and makeup of the area, if memory serves me correctly just east of Gameracha heading to Birdwood around the 2 Km mark there is a lovely pegmatite that crosses the course of the River Torrens, I recall finding some well shaped crystals of green Beryl on the hill beside the pegmatite a long time ago. Good Luck in your quest.
Pete