You can find onix in the cave which takes you from the 1st gym town to the 2nd, sorry for not knowing the name, my game is frozen :)
(second addition to answer)
Go to Violet City...There is a house in the bottom Left of the town. The Nerd with the glasses will give you an Onix if you trade with him...The Onix's name will be Rocky, at it will be at whatever level the Pokemon you traded him was.
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Onix > trade w/ Metal Coat > Steelix
There is no such thing as a Crystal Onix. It only exist in the anime.
In Pokemon Yellow Onix does not evolve, it is a one stage Pokemon.
If you mean,, what onix evolves into,, it's steelix,, but the is no Pokemon that evolves into onix
Unfortunately, you can't get an Onix in Pokemon Emerald. To have your own Onix, you need to trade it from FireRed or LeafGreen. Hope I helped :)