Nothing To Scry About

You should hoard these stones and wait till you get flying if you want optimal efficiency when attempting this achievement.

If you do that, you can do the achievement using only 2-5 scrying stones.

I did it as a druid with flight form and was very easy to complete this way. Make sure you hover close to the ground so you can use your extra action button. Also a macro for extra action button use is recommended for this achievement. Bind it to a key that is easy to use.

/click ExtraActionButton1

A good place to start is at the Drowned Market, then proceed to the Coral Forest. Dragon's Teeth Basin also has a good density.

/way 77, 28 Drowned Market
/way 61.9, 51.7 Coral Forest
/way 46.2, 47.5 Dragon's teeth basin

You can pretty much search anywhere in the zone, most places have plenty of chests in close proximity to one another.

Keep an eye out for the [spell=305022] buff, which allows you to see chests on your minimap. Once you have this collecting becomes very easy (with flying).

There are plenty of areas with a high density of chests. Ensure you are opening as many Glimmering Chests as possible so you can keep refreshing the Scrying Stone buff. If you're lucky you can also get reputation items from these chests (150 rep tokens which are unique)

Edit: Blizzard implemented a hotfix which ensures that every chest counts towards your progress. Now is the time to finish this up.

Edit 2: [item=152813] removed from comment. Not as useful for other classes since they can't use extra action button when mounted. My apologies, wouldn't know since im a druid, heh.

Happy hunting!

Using the [item=167077]:

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• Collect at least 2-3 [item=167077]s before you start.
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• Click on 1 [item=167077] at a time. This generates the [spell=300135] buff with a 5 minute timer shown in the upper right of the screen with other buffs.
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• Run around (or ride on the ground) until a [item=167077] appears in lower-mid screen. Stop. Click on the icon to spawn a chest. The mini-map will show a single yellow dot marking the location.
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• The blue chests are the same model as the arcane puzzle chests. They sparkle and they’re not hidden.

Sometimes you get the [spell=305022] buff which fills the mini map with yellow dots.

• Each dot shows the location of a chest — with a catch. When you reach one of these dots, stop. You won’t see the chest, you’ll see the [item=167077] icon in lower-mid screen. Click on it to spawn the chest. Check the mini-map to confirm the exact location. Move quickly to the next dot.

Want to pick a good location?

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• 
  ruthven78 suggests looking at the map for [object=322413].
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• (Drowned Market is one place with fewer mobs.)
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What else?

• Keep an eye on the time remaining on the [spell=300135] buff. Sometimes the time will be extended when you open a chest.
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• Be prepared to stun/fear local trash. Bring mudwraps, totems, etc.
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• If you run out of [item=167077]s, go kill a rare. [item=167077]s are dropped by 59 rares. See the "Dropped by" list (scroll down). Out in the field, look for npc's with yellow stars, check the Nazjatar war board or find a group farming rares.
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What's all this strange stuff I'm looting from the chests?

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• If you loot a green level 1 item with no apparent purpose, it can often be traded at your base with the murlocs. It will say it lasts less than 24 hours.
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• On [npc=152084]'s home page see Dioxina's amazing table (scroll down) showing items that can be swapped with murlocs. Many items require multiple trades with different murlocs. A few items can be purchased for coin.
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• To see [npc=152084]'s hidden wares, buy and wear the [item=169489] during your transaction.
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"Treasures" is the key word here... Just opening100 chest isn't the objective, finding 100 "Treasures" is. Only time my achievement has moved is when I've looted an epic quality item.

EDIT: Got 5 Manapearls in 1 box, counted as 5 "Treasures".