November 2026 – Cristobalite

Behold – the Black Abbott!

Or is it just a piece of obsidian? Never mind, the main thing is that it’s perfect for both Halloween and a grey November. To go with this, we’ve chosen a very special mineral for you – Cristobalite. Cristobalite, much like quartz, is a form of silicon dioxide (SiO₂), but is only stable at much higher temperatures, above 1470 °C.

Nevertheless, it can form at significantly lower temperatures – as seen here as a spherical aggregate within obsidian. Obsidian is a volcanic glass; it forms through the ‘freezing’ (very rapid cooling) of silicon-rich lava. This glass is quite unstable, which is why it tends to undergo ‘de-glassing’, a process during which, crystalline phases like cristobalite form.

Chemical formula: SiO₂

Location: Superior, Pinal Co., Arizona, USA