Dragon Age Inquisition bug

If the diamond is faded it means that the objective is on a different elevation than you, so it could be above or below you. Otherwise it may may glitched into the ground, so you may have to slowly wave your cursor over the area to see if anything can be interacted with. I believe in one area the diamond was for something just on the other side of a wall - took me a while to get to it.

But if you are looking for the Heart Of Despondancy, here is a walkthrough for the artifacts in the area:

http://ca.ign.com/wikis/dragon-age-inquisition/God_of_Secrets

In the video in the walk through, he enters the Chamber of Despondancy just after the 20 minute mark and gets the heart at 20:34

And for the other quest in the location:

http://ca.ign.com/wikis/dragon-age-inquisition/Runes_in_the_Lost_Temple

PS You really should keep a minimum of 5 saves going for problems like this. Unfortunately the game can glitch out and you won't be able to move forward from a current save, and the best chance is to reload an older save. Otherwise you may just have to abandon the quest. But it doesn't prevent you from finishing the main story.