Need help understanding some things about sealing an injector

[gdyer101]

I'm sorry if my knowledge on the subject is very basic, but I could really use some help. I'm actually swapping my pressure injector for an EFI and am simply trying to understand the old one before installing the new one. Attached below is a picture of the old injector from a diesel Yanmar test engine. I'm curious as to what the black material is near the tip of the injector and what its purpose is. It feels like plastic or rubber. I assume it's ceramic since plastic/rubber wouldn't be able to withstand the heat so close to combustion. I'm thinking it might be some sort of sealing method since to my knowledge there wasn't a copper washer in this system. these are just theories however, as I don't have much experience with the subject. Any knowledge as to what this is or what it's for would be greatly appreciated. My main concern is sealing the new injector, so I am trying to figure out how the old one worked. Thank you!

[brian617]

You sure the copper washer isn't still down inside the injector bore? I have a small John Deere tractor with Yanmar 3 cylinder diesel and it uses copper sealing washers.