You will need to fall through the ice (walk on it a second time and it will collapse) and work your way around until you get to the HM for Waterfall, teach a Pokemon that and you can then proceed with the hunt for the Sapphire.
on Island #4 in icefall cave icefall cave
Icefall Cave is where you get 1 of the gems for Celios machine on island 1.
Four Island
It can be caught in the inner cave of Icefall Cave. It has a 5% chance of appearing.
icefall cave
icefall cave firered only
It is in Four island
In icefall cave.
Inside icefall cave.
In icefall cave.
icefall cave on island 4
Four Island, Icefall Cave.