The word "adamant" derives from the Latin word adamas, a hard stone. Over time, it came to refer to anything that was not easily broken or changed. Today we use "adamant" figuratively to describe someone whose opinion can't be changed, as in "He adamantly refuses to go to his in-laws’ house for Thanksgiving." 

However in Shakespeare's time, adamant was used as a noun and had a very specific meaning. It referred to a lodestone, a natural occurring magnet, and that's how Helena's using it here. She's attracted to Demetrius, like a piece of metal is to a magnet.

Today, magnets are very common because they are easily produced by exposing a piece of metal to a strong electro-magnetic field, but in Shakespeare’s day they were quite rare because they only occurred when lightning happened to strike a stone containing iron ore.