Hi.

Why a diamond needle wears out by playing vinyl records ? What is there in vinyl harder than diamond in order to erode it ?

Having no idea what I am talking about I would assume that the diamond:

1. might wear due to its small size

2. might fracture

3. it's something other than the diamond which wears out

a. the metal holding the diamond

b. the arm holding the diamond assembly

c. isn't there a small coil also on the needle?

The diamond actually wears, it has less to do with how hard the Diamond is vs the Vinyl as it does about how much vinyl rubs the needle. After many miles of groves even a diamond will erode. There is no doubt the needle eroded many times it's weight in vinyl but ultimate the vinyl won. It's like water eroding rocks, lots and lots of water and a little bit of rock is eroded, over time the water wins. And yes the vinyl will eventually be worn out as well.

i'm sure if you played the same record over and over again till the diamond needle wore out you'd end up with a coil of vinyl instead of a disk long before the needle even got blunted.