I actually did a tiny bit of reading on agave nectar and it seems a though it actually helps to balance sugars to avoid that high to low feeling, like pricky pear does, and is actually recommended for diabetics.
I wouldnt use it as a substitute, but as a licky treat is fine. I sometimes mix it with acacia powder as a treat as well.
http://www.allaboutagave.com/coudnt find nutritional data for calcium in the nectar itself, but actual agave is VERY high in calcium