Once you have completed all the free quests in Runescape nothing happens, you can continue to play as normal and if you have members benefits then you can carry on your questing by completing members quests.
You collect gems for free by completing quests or clearing obstacles from your village.
No. You get a free pass from completing one of the beginner quests, though.
By completing Quests
By completing some quests. That's where I got some Cafe' Life gems. Even coins, too.
These can be obtained by completing Dage the Evil's Champion Quests, the Paragon Pet's Quests or Rogath's Quests.
Seaside stickers can be obtained by completing quests given in Seaside. The stickers are only given out during special events.
SMD
You get diamonds by completing the 'knutt knight' quests xx
He is accessed by completing enough quests for Dean Warlic.
Don't use it up and you get it by completing quests
completing quests/tasks given by NPC's. you will also get rewards for completing them.
You get the armor by completing the quests in the Knights of the Nine official plug-in.