Thief 3

The ever so sly and cunning theif sneaked into the museum which holds the worlds finest jewels.He wore a black mask whcih covered his entire face except for his eyes.He was wearing a suit as black as charcoal and had dark blue sapphire eyes.In one hand he held a big empty sack ,ready to be filled with the finest jewellerys there is.As he walked into the hallway he saw the most magnifecent ,beautiful and biggest diamond ever! kept in a glass box. He made a wicked smile ready to steal the diamond,reaching out with his enormous hand holding a knife ready to cut through the glass box and steal the diamond.

The ever so sly and cunning (theif) thief sneaked into the museum which (holds) held the world's finest jewels. He wore a black mask which covered his entire face except for his eyes. (He was wearing a suit as black as night and had dark blue sapphire eyes.) In one hand, he held a big empty sack ready to be filled with the finest jewels of all colours.

As he walked into the hallway, he saw the most (magnificent, ) eye-catching and biggest diamond ever! It was kept in a glamorous glass box.

He (made a wicked smile to himself) smiled wickedly (and got ready to steal the diamond,) reaching out with (his) an enormous ( hand holding a) knife, ready to cut through the glass box and steal the diamond.

Mrs Chien: I can see this thief! This means the description was effective! Yes!

Now, sometimes, it's better to refrain from physical descriptions as they can sound awkward. Instead describing people's actions is more interesting. Also 1 brilliant adjective can be better that 2 not so great ones.