By catching all the unkowns after finishing the Pokemon league
It's in Mt Ember inside a cave Rocket Grunts opened for you.
You can find one in the Icefall Cave on Four Island.
Once you beat the elite four and get the national dex head to one island and go north to mt ember in there two rockets opened up a cave battle them then go in the cave in here use Strength to find the ruby!
you get one in icefall cave on four island
On seven island near tanoby chambers.
You can find one in lost cave on five island.
the ruby is in the cave on mt. ember on one island. the sapphire is in dotted hole on six island.
On island one, there is a cave with a ruby and a full set of braille alphabet.
you have to have beaten the elite 4 and givenh celio on island one the ruby you go to island 4 and find hm 7 waterfall in waterfall cave
you can find lapras in leaf green or fire red. there is one in sylph co on the seventh floor or you can find one on four island in icefall cave but they are EXTREMELY rare
You can raise one up from a zubat or it can be found in the victory cave on the island where Ever Grand City is.
Answer:6 Island.