What are some eight-letter types of rock?

13 of the 78 types of rock in Icillion have exactly 8 letters, for example Obsidian, Rhyolite and Andesite.

Which eight-letter types of rock start with M?

M is the most common first letter here, with 2 names, such as Mudstone and Mylonite.

Science · lists

13 eight-letter types of rock in alphabetical order, from “Andesite” to “Trachyte”. Each one counts as an answer in Icillion.

13 of the 78 types of rock have exactly 8 letters (17%).
Most start with M (2), P (2) and T (2).
Only names of one word are counted, and only their letters: spaces, hyphens and apostrophes are not.

What are some eight-letter types of rock?: 13 of the 78 types of rock in Icillion have exactly 8 letters, for example Obsidian, Rhyolite and Andesite.
Which eight-letter types of rock start with M?: M is the most common first letter here, with 2 names, such as Mudstone and Mylonite.

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