Wombat, as the daughter of a gun nut who made his own bullets (there's an area of my life I don't think I've touched up on here much!), I can clear this up for you. The bullets you see in jewelry are empty - they don't have powder in them, so there is no element in them that could cause explosion. My dad thought he was being pretty cool with his hollow bullet casing necklaces and key chains and such, but... um, no.
When it comes to an actual, full of powder bullet, rather than an empty bullet casings used for "decorative purposes," yes, if you drop a bullet, point down, onto something hard enough like a concrete floor, it could explode on impact. I've seen it happen once, but this occurred when the bullet was intentionally basically thrown at the floor, point down. Likewise, I've seen way more bullets roll off a table and bounce around on the floor without anything bad happening, but apparently it was super dangerous anyway, and lead to one parent yelling at the other, and me closing my bedroom door for the 100th time that week.