Question

i have never seen such a big diamond as this. transform in superlative degree

Answer 1

this is the biggest diamond i have ever seen.

Answer 2

"This is the biggest diamond I have ever seen" is the superlative degree transformation for "I have never seen such a big diamond as this."  

An adjective has three forms of degree: Positive, comparative and superlative.

Superlative form is the highest degree of adjective that is generally used to express some extreme situation.