In the manga, Rei is often shown wearing a necklace. In fact, during the earlier arcs, she’s never seen without it.
The Materials Collection even makes special mention of it.
It also mentions that, after transforming into Sailor Mars, the pendant moves to her waist and becomes a tool that helps her generate fire.
To me, this seems to imply that Rei’s pendant has a special significance to her. So I did a little digging through the manga, and lo and behold:
That’s right; Rei’s deceased mother, Risa (name revealed through PGSM, btw), owned the exact same necklace! See?
So my idea is that, before she died, Risa passed her necklace onto Rei and Rei’s treasured it ever since. She treasures it so much that it actually becomes a part of her fuku and helps her with her powers; in a way, it’s almost like Risa herself is supporting and helping her daughter even though she died years ago.