You need Tinkering 100 to craft the necklace itself. Beyond that, there’s no skill or stat requirement to unlock slots or install any gem – unlocking is purely based on your account’s Mastery Level.
No. You earn Mastery experience just by dealing damage to and killing monsters, whether or not you’re carrying a necklace at all. You only need an active necklace to actually receive its gems’ bonuses.
Yes. Your Mastery Level and experience are account-wide – every character you play contributes to the same total, and every character benefits from the level you’ve reached.
No. Gems live on the specific necklace object you socketed them into. If you craft a second necklace, it starts empty – your account-wide level just decides how many of its slots are unlocked.
No. Only one Gem Mastery Necklace can be active on your account at a time, and switching which one is active has a short cooldown.
The necklace becomes bound to your account the first time you activate it or socket a gem into it. After that, only your own characters can use it – anyone else who picks it up gets nothing from it. A never-activated necklace isn’t bound yet.
Your necklace itself is protected and won’t drop on death. Loose gems sitting in your backpack are ordinary items and follow the same death rules as anything else you’re carrying, so keep valuable gems banked or socketed rather than sitting loose in your pack during risky fights.
No. Removing a gem is free, and the exact same gem (same material, same bonus) is returned to your backpack.
No. If you want a different bonus, remove the gem you have and install a different one.
No. All Mastery Chain bonuses – including follower bonuses – are switched off while you are in PvP combat, and stay off until your PvP combat timer runs out.
No. Regular Halloween activities like trick-or-treating, candy shopping, and decorating don’t grant Mastery experience or gems. Three specific Halloween quest lines – the Trick-or-Treat Trail, the Headless Search, and the Ghost Tour diary – are an exception, each granting a one-time experience bonus the first time you complete them. See Earning Mastery.
That depends entirely on how you play. Melee fighters, spellcasters, tamers, bards, and gatherers all have bonuses aimed at them – see Gems and Bonuses for the full list and pick what suits your character.
Yes, but every bonus type has a combined ceiling of 35%, no matter how many gems you stack into it.
Neither is sold by vendors. Craft the necklace via Tinkering, and find gems as monster loot.