Lately I have been making regular Posts in the Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”. This Group, with currently just over 4,000 members, is described by its organisers as follows: “The Group is for anyone interested in rockhounding, collecting or any of the rock Lapidary arts. Its focus is on New Zealand material but anyone can contribute overseas material if you worked it here in New Zealand.” It is very useful for someone like me to have a Group that has a local focus. A number of members have expertise in different regions of the country or in different types of rocks. It is also great to get reactions to my own material and ideas, and encouragement from people who share the same passion/obsession.
Some of the Posts I have made recently in this Group are about stones that I have just finished tumble-polishing, other Posts draw on material from TumbleStone Blog, and some relate to current interests of mine. Here are three from early this month (apologies to those who have already seen them before).
Sunday 2 May 2021: Small (3 cm) jasper stone recently tumble-polished, found on Gemstone Beach, Southland, on 28 February this year. Maybe some green epidote also present.
The stone looks like what Jocelyn Thornton refers to as a hematite jasper (page 36 “Beach Pebbles – Orepuki” in “Gemstones”), because of the grey material that the dark red is embedded in. The bright specks in it seem to be the light glinting off the metallic planes. They could be small particles of polish remnants but the stone spent two days in borax after polishing. Also, checking with photos of the stone before it was tumbled, some of the bright glinting seemed to exist at that stage. However, the stone is not metallic – to my surprise, it does not respond to a magnet, so it is highly likely not to have hematite in it. [I later discovered that hematite is very weak magnetically so this stone is very likely to be a hematite jasper.]
Monday 3 May 2021: Small (3 cm) stone recently finished the tumble-polishing process. Found on Gemstone Beach, Southland, about a year ago. It appears to be a brecciated stone, made up of many tiny pieces in a fine-grained matrix.
“Breccia” consists of pieces that have not been rounded much by transport by water, for example, before being cemented together; “conglomerate” consists of more rounded pieces indicating some erosion before being cemented together and thus have travelled further before formation. The fragments may be the result of faulting (even small movements in rocks caused by pressure) or a landslide or something that causes fast erosion. Of course, some stones can consist of both sharp-edged and rounded fragments, confusing the categorisation! One of the potential “problems” with polishing such stones is that different embedded fragments may polish quite differently and some may not polish at all. This stone has polished much better than I expected, despite its complex make-up, with only a couple of tiny un-polished “dots” on one side.
Tuesday 4 May 2021: Recently tumble-polished small mudstone (3 cms long) from Gemstone Beach, Southland.
Often called a “faultline”, “faulted” or “earthquake” stone because of the displacement captured within it. I think of it as an “offset stress fracture” due to the very minor movement that has been frozen in time. Not an unusual type of stone – I have come across a few – but it does attract my attention when I see one of these. This little stone has tumble-polished well, ending up smooth and relatively shiny, though it hasn’t taken a high polish.
See here for the next Post in this Series.