Razor sharp means as sharp as a razor. :P So they can't cut diamonds - unless you know something I don't? 

- They're sharp enough to cut through flesh and strong enough to break through some bone too if hit correctly. They can extend and fully contract, but always grow from the chest. So it's much easier for him to defend or attack something in front of him. 
- When used to shield against something, together they're only wide enough to protect about 90 degrees in one direction, or 45 degrees in two directions.
- They're strong enough to protect from weaker ranged weapons, but rifles, snipers, explosives and any well angled shots destroy or surpass his tentacles. 
- They could be used to climb over obstacles like arms, but again only obstacles of a certain height, and doing so would leave him unable to shield himself. 
- They're completely invisible. He can sense them like an amputee senses a phantom limb and knows where they are, although feels no sensations from them.