After riding a Rhyhorn along Route 9: Spikes Passage, you arrive at the cave entrance of Glittering Cave.
Explore the Cave
In this cave, there are no random encounters, but you may find a shadowy spot in the cave. The shadowy spots are wild pokémon that you must battle when you walk up to it. You can encounter Cubone, Machop, Onix, Rhyhorn, Lunatone, Solrock, and in rare cases, Mawile or Kangaskhan.
Walk straight ahead until you reach a fork in the path. Go to the right. At the end of the path you encounter a wild pokémon.
Now go back and turn right at the fork. When you reach another fork, go left. There is another wild pokémon at the end of the path.
After you are done battling the pokémon, walk forward until you reach a dead end, where you will find a Hard Stone.
Go back and then go left at the fork. Follow the path and you will encounter a wild pokémon.
After defeating the wild pokémon, go straight ahead until you reach another fork. Go left. You encounter another pokémon.
After defeating this pokémon, go straight and follow the path to the end. You encounter another wild pokémon. After defeating it, you can reach the TM65 Shadow Claw that was behind it.
Now turn around and go back the way that you came. Go left at the fork and continue along the path. Go into the cave entrance at the end.
Meet Team Flare
You end up in a room. In this room, watch out for shadows from the ceiling, because if you walk under a shadow, a Woobat or Ferroseed will fall and battle you. You can also encounter wild pokémon or find fossils if you use Rock Smash on the light brown rocks here. You can get the Rock Smash TM in Ambrette Town (where you just came from).
Get ready for battle and walk north. You meet a mysterious figure dressed all in red. This is a representative of Team Flare, a fashionable group of villains who want to be the only ones in the world who are happy. The Team Flare person tells you to leave. If you're ready to start the battle, say No. Otherwise, say Yes if you want to prepare first.
The Team Flare Grunt has a level 18 Houndour and a level 18 Zubat.
Go east along the train tracks. You'll have to battle another Team Flare Grunt. This one sends out a level 18 Gulpin, followed by a level 18 Electrike.
Continue along the path. Eventually you reach two Team Flare Grunts. Your neighbor appears and asks to battle them together. Say yes if you are ready.
The Team Flare Grunts send out a level 20 Croagunk and a level 20 Scraggy. They apparently want to make lots of money to achieve their goals, and they hope to make some money by finding fossils.
Continue past the two Team Flare Grunts. To the south, you find an Escape Rope.
Go talk to the researcher north of where you found the Escape Rope. Your neighbor will join you. The researcher was so busy looking for fossils that he didn't even know Team Flare was there. He offers to give each of you a fossil. The Jaw Fossil can be turned into a Tyrunt (Rock/Dragon Type), and the Sail Fossil can be turned into Amaura (Rock/Ice Type).
After you choose your fossil, your neighbor suggests that you challenge the Cyllage City Gym. Use an Escape Rope to leave the cave quickly. Ride the Rhyhorn back to Ambrette Town.
Revive the Fossil
Back in Ambrette Town, go into the Fossil Lab and talk to the researcher behind the counter to revive your fossil.
Visit the Aquarium
You can reach Cyllage City by going through the Ambrette Town Aquarium. The Aquarium is next to the Ambrette Town Pokémon Center.
Be sure to talk to the fisherman in the downstairs area of the aquarium. He gives you an Old Rod, which allows you to catch new types of pokémon that you couldn't get before. To use the rod, just walk up to some water, open your bag, select the rod in the key items pouch, and use it. If an exclamation mark appears, press A to reel in the pokémon. If you don't press it quickly enough, the pokémon will get away. There are fishing spots in places that you have been to before, like Santalune Forest and Parfum Palace, but you might want to wait until you can go back there more easily.
When you are ready, go outside to reach the lower part of Muraille Coast.