The 1.0.2 patch will go live next week, and includes several changes to the Cryptarch and Engram experience. These changes guarantee that Engrams will always decode into items at or above the quality of an engram.

Only Engrams that drop after 1.0.2 are subject to this change.

Cryptarch Changes
Cayde-6 took the Cryptarch aside and showed him a sack of doorknobs. He decoded that mystery pretty quickly.
   - Legendary Engrams will always produce Legendary or better quality items, including Materials or Exotics
   - Rare Engrams will always produce Rare or better quality items
   - Rare engrams will have an increased chance to produce Legendary quality items

Activity Changes
   - Daily Heroics, Weekly Heroics, and Vanguard: Tiger Playlist activities will include Rare and Legendary Engrams in addition to their existing rewards

Item Changes
  -  Ascendant materials have been promoted to Legendary to closer associate them with the gear they are used to upgrade
  -  Legendary Engram items that exist in your inventory will be demoted to Rare quality when the patch goes live, so decode them while you can. But let's be honest--even if you don't, we all know they were blues already...