33mm x 25mm
34mm x 25mm
I call Usan agates like these "Black Agates" even though they are actually grey in colouration. The term seems apt as from the outside, the water and gravel-worn nodule will usually show staining in black. It appears that these particular agates, upon erosion from the andesite, became buried in the dark Usan gravels that predominate in certain areas. Immersion in this gravel has resulted in the agate becoming a dark blue-grey colour. The banding is intricate in these specimens, and in this piece appears like ripples in calm water. Layers of banding can be observed behind one another. Another distinctive aspect of this type of stone is the detailed and textured tube of escape (seen here in the middle top-portion of the face)
In nearly 4 years of collecting in the area, I have found only 4 examples of this type of agate. Interestingly this particular specimen was found in an area where I have found no other examples. However the 2nd and 3rd examples were found about 12 metres away to the north in a gully, and only a few metres apart. The mercurial Usan tides reveal them from time-to-time and most recently (April 2016) I turned-up an example that I could see from a fair distance away such was its distinction. That stone is the largest of its type I have seen (still under 4cm) Another possible term that could be applied to these stones was "Ghost Agate" because of the swirling blurring and intricate banding.
It appears then that "Black" Usan agates originate in offshore shingle, which is obviously inaccessible. Diving in the area is treacherous, and the water is frequently very cloudy or filled with seaweed torn-up from the depths. Perhaps if one was able to dive in the area during a low tide when the wind was right it might be possible to uncover agates in offshore areas.