Good post to spend a bit of time on going through my Rocks and Minerals book. Without the benefit of hands on my best guess is maybe a fine grained Peridotite (Kimberlite). Peridotite is the main rock of the earths mantle and when exposed on the Earths surface it is usually coarse grained due to the eons of gradual cooling and weathering needed for its exposure which allows for the growth of larger crystals.
Occasionally when it is erupted much faster in a volcanic event, Peridotite assumes a much finer grained appearance due to more rapid cooling and is then referred to as Kimberlite.
Kimberlites are the source of most of the diamond deposits of the world.
The attached image shows a Kimberlite rock with some mineral and diamond enclosures.
I believe Diamonds have been found in the New England region of NSW before.
Maybe crack open a fresh face and look for any enclosed minerals.
Good Luck.
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