It would have been a terrible idea for Voldemort to use random objects as horcruxes...
Maybe I'm stating the obvious here. I may also be wrong...
This is a response to the people saying "Why not use a random stone on the beach?" or words to that effect. I think this comes from an easy misunderstanding of what a horcrux does. If you have a horcrux, you are not invincible or even really immortal. Harry could have shot Voldemort and killed him and that would have practically killed him.
Horcruxes are backups. If Voldemort is killed, his followers could have made another one. This is what nearly happened with the diary and why multiple Voldemorts was a possibility.
So it follows that his most trusted followers need to know what the horcruxes are and how to find them. It makes sense that they have to be recognisable at a glance.
Granted, it is funny to think of a hoarde of Death Waters scouring a pebbly beach to determine which pebble has the most evil vibes...