4 words for MNX 1–2 letters

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1 letter (3)

m n x
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2 letters (1)

mn
Form of quartz with coloured bands
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Hint 1 meaning
A semipellucid, uncrystallized variety of quartz, presenting various tints in the same specimen. Its colors are delicately arranged in stripes or bands, or blended in clouds.
Hint 2 anagram
TEGAA
Hint 3 another clue
CERTAIN BRAIN SIZE
4 +1