“... These are from Otago Province which is further South from where I live. They are seam agates, or at least the remains of them and they come from the area around Moeraki and Hampden about 30 miles south of Oamaru. Most agates from this area are found on the beaches, but some unusual black specimens can be dug from the middle of an estuary/swamp. The best time to dig is when the water has built up and forced its way out through the sand dunes leaving the ponding much shallower.
It is quite common to find small sagenite needles and black dendrites in the beach agates....”
personal report from Malcolm Luxton, Canterbury