Onyx sits next to 4 phonetic neighbors — Knox, Phoenix, and Felix are the closest matches by sound. Most share Onyx's Greek (onyx)-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"black gemstone"
"red-haired, little red one"
"friend, companion"
"unity; the best; one"
"lion"
"hillside dweller"
"meadow land"
"free, liberated"
"money box, treasury"
"eternal ruler, ever powerful"
"kingly, of royal blood"
"of the Julian clan, youthful"
"son of Jens (son of John)"
"warrior, man"
"at the ford"
"meadow dwelling, water meadow"
"battle (possible Irish Gaelic root)"
"slender"
Similar sound: These names share phonetic patterns with Onyx — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Greek (onyx) origins, giving them the same cultural heritage as Onyx.
Rhythm: Onyx is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.