User blog comment:CNBA3/Fairy Tail 412 Review/@comment-7418318-20141208124032/@comment-25111584-20141209120844

I always thought that the lacrima were containers of Magic which were given their respective Dragon Slaying properties by Rogue and Sting. Enhances their Magic Power and helps them have better control over their Magic rather than anything else, as they can already use Dragon Slaying Magic. Natsu, Gajeel and Wendy might just never have thought of it or known that it could be done.

Ivan and Cobra could have somehow managed to lay their hands on lacrima created by former Dragon Slayers. For non-Dragon Slayers, they could enhance their Magic Power as well as allow them to use Dragon Slaying Magic.

All speculation though. >.>