The Sapphire is found in the Six Island Dotted Hole, which is in the Ruin Valley south of the Water Path. The order to fall down the holes is:
Enterance: Fall Down only hole.
The SApphire is on B5F. A guy will steal it, and you must go and get him at the Rocket Hideout on Five Island, in Five Island Meadow. You must go through the puzzling Hideout to get to the guy who stole the Sapphire. Battle him, and win... He will give you the Sapphire.
on LG how do you beat loreli
Loreli's Poke'mon can be defeated by the following:
Dewgong Level 52: Electric or Fighting Type attack (preferably one with high speed, like Jolteon)
Cloyster LEvel 51: Fighting or Rock Type (Cloyster has a high Special Defense and Defense, but physical attacks work best)
Slowbro Level 52: Dark or Electric (Do Not use Grass or Bugs against it, it does know Ice Beam)
Jynx: Level 54: Fire, Dark, or Rock Attacks (Preferably Fire Type attacks
Lapras Level 54: Electric or Fighting type (Preferably a high speed electric type)
And most importantly, make sure you have at least 6 Hyper Potions for battling her... And make sure you save before even entering the Poke'mon League's doors to challenge the Eliete Four.