Tablets

Tablets are a useful item that can be used only once each to permanently increase base stats of your character. However you must use them in order, gray before green, green before blue, etc.

http://warofdragons.com/artifact_info.php?artikul_id=13722
http://warofdragons.com/artifact_info.php?artikul_id=13727

Note: Some tablets start at blue so youwon't find gray or green. These are Wisdom, and Mana as they can be used by mages only.

Tablets can be bought on from other players via auction, exchange and trade or made from fragments which you can find in blue sacks.

Blue Sack

You can buy them during Trade Fair for 30s, for 0.22 diamonds in the Premium Shop or for 20 Merchant Marks in the Exchange Tent in Ciy Fair. (For more information on Merchant Mark exchange options click here.)

Note: Most of the bags will only contain fragments not a full tablet, though it is possible.

In the likely event all you got was fragments you will need a certain number of them to make a random tablet. 

Level restriction: To use ceramic tablets you need to be at least level 6, for earthernware ones at least level 11 and level 16 to be able to use the porcelain ones.

In your attributes there is a section for "Ancient Knowledge" to see your current bonuses from the tablets.

You will also get an achievement for using a new tablet. You can see them all under "Guidebooks" in the achievements section.