Some stones consist of a number of different substances, and not all of the substances may take a polish or polish to the same degree. Stone #26 is a good example of one such stone:
The grey material has not polished well but the small inclusions set within that grey material have:
I don’t know what kind of material makes up this stone. On the beach, the bright inclusions against the grey background caught my eye.
Stone #26 is 4.5 cms long, 3 cms at its widest, and about 0.5 cm thick. It completed the polishing process three days ago. It was polished in a 3lb barrel along with another 56 stones from Kakanui. All the stones had undergone tumbling in 400 grit before spending 11 days in tin oxide tumble polish and three days in a borax burnishing tumble. The tin oxide mix had been used four times before (I usually re-use the mixes between six and eight times – six is the recommended number of times – I always record each time I use a mix and for how long). As can be seen in the photo below, Stone #26 stood out as the only stone that did not take a reasonably high polish.
Note: Jo from the North East of the UK posted a comment on a photo of this stone: “January 24th is my birthday and this year I went pebble hunting on our local beach… and found something very similar which I think is a Rhomb-Porphyry from Oslo.” According to Sandatlas, rhomb-porphyry is “a porphyritic igneous rock [andesitic] with abundant wedge- or lens-shaped anorthoclase (feldspar) phenocrysts. Rhomb-porphyry is a rare rock type. The most well-known is the rhomb-porphyry from the Oslo Rift in Norway. Similar rocks are known to exist in only two other locations: The East African Rift Valley and the Antarctic.” Stone #26 certainly looks similar to photos of rhomb-porphyry (a “rhomb” is like a “pushed-over square”, where all sides are of the same length, though the phenocrysts in examples tend to be of a variety of shapes):
Whether Stone #26 is a “rhomb-porphyry” or not would require a geochemical analysis. Jo has made a very interesting suggestion which has led me to find out more about the great diversity of stones on the planet.
The Index to the January 2022 Stone of the Day Series is here.
3 thoughts on “January 2022, Stone of the Day #26 – Grey Kakanui Stone with Shiny Inclusions”