I cannot recall the name of this anime. It's about people who can, with the help of some sort of crystal, get these bionic suits in which they fight against aliens with like yellow blood, and a group of three who want to extract the core from these aliens. The bionic suits of these three are also yellow.
Is it The Guyver? I'm not particularly familiar with the story, but it does involve a crystal that gives you some kind of armor and powers. The bad guy aliens if memory serves were after the Guyver in order to get its crystal.
I got it - its name is Hundred.
When an extraterrestrial organism known as "Savage" attacks mankind, the only technology capable of combating the enemy is a weapon known as “Hundred.” A survivor of a Savage attack, Hayato Kisaragi is a teenager boasting the highest compatibility level with the aforementioned technology and as a result, is invited to master his skills at Little Garden, a prestigious military academy aboard a battleship.
Over the course of his intense training for the battle ahead, he immediately attracts the interest of multiple female peers and gets drawn into a number of incidents as he tries to fight against the creatures that now inhabit Earth and threaten its safety. - Synopsis from MyAnimeList