Orbs for Armor Upgrade
Gemstones like that can be used to upgrade anything with an open slot. However, the gear you are upgrading must be a high enough level to accommodate that gemstone.
Orbs (highest tear gemstone) can only be put in level 80 gear. It’s probable that your body/leg/arms are currently level 80 gear, and your other peices of armor are lower level.
I just looked at the Wiki page for the Ruby Orbs, as I know/knew nothing about them. Are all your armor pieces level 80? The orbs seem to have a level 80 requirement.
That explains it. Thanks a lot. = )