The white “dots” in Stone #12 caught my eye when I spotted it on Gemstone Beach. Close inspection revealed well-defined rounded crystals – or are they worn fragments of quartz (making this a “conglomerate” stone)? This is sometimes a puzzle with stones that look like this – it can be hard to tell whether they contain crystals or fragments unless you have some appropriate geological knowledge.
My eye was initially drawn to the round white quartz in the stone. However, after looking more closely at the polished stone, I noticed crystals or fragments of something else there as well, having a rough silvery-gray appearance.
Conglomerates are similar to breccia in that they both consist of fragments of rocks cemented together in a fine-grained matrix. The difference between them is that the fragments making up conglomerates are rounded and worn while those making up breccia are angular and r0ugh. Geologyscience.com has an excellent page on conglomerates, including how and where they form.
Go to Stone of the Day #13.
Go to start of the Stone of the Day Series for November.
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