Icon of the Silver Crescent

Source
You can buy this item from G'eras in Shattrath City for 213 Justice points.

Effective Damage
Assuming you use this Trinket every cd, it will be active for 20s out of every 2 minutes(i.e. 1/6th of the time). This means you gain:

158 * (1 / 6) = 26.3333 +damage

Giving a total effective +damage of:

26.3333 + 44 = 70.8333 +damage

(May be outdated as of Patch 4.0.1.)