A ‣ Positive and negative qualities

positive
She has a heart of gold. [ very kind, generous]
He's as good as gold. [helpful, well-behaved; used generally for children)

negative
She's as hard as nails. [no sympathy for others]
He's a nasty piece of work. [unpleasant]

Note also:
Her heart's in the right place. [is a good and kind person even though they do not always seem so]
He's such an awkward customer. [difficult person to deal with]
She's a pain in the neck. Nobody likes her. [nuisance, difficult)
He gets on everyone's nerves. [irritates everybody)

B ‣ Idioms based on ‘head’

You can learn idioms by associating them with a key word or words. Two of the idioms in A, for example, are based on gold and two on heart. Here is a set of idioms describing people based on the word head.

to have your head screwed on [be sensible, informal]
to have a head for heights [ not suffer from vertigo]
to have a head like a sieve [bad memory]
to have a good head for figures [be good at maths]
to have your head in the clouds [unaware of reality)

to be head and shoulders above someone [much better than)
to bury your head in the sand [refuse to think about a difficult situation in the hope you won't have to deal with it)
to keep your head [stay calm in a difficult situation]

C ‣ How people relate to the social norm

She's a bit of an odd-ball. [peculiar, strange]
He's really over the top. [very exaggerated in behaviour]
He's (gone) round the bend, if you ask me. [absolutely crazy/mad]
My politics are very middle-of-the-road. [very normal; no radical ideas; neither left- nor right-wing]

D ‣ Who’s who in the class? Idioms for people in the classroom

Sam’s teacher’s pet. [teacher’s favourite] Laura’s top of the class.
Harry is a real know-all. [ thinks he knows everything]
Ali's a bit of a big-head. [has a high opinion of him/herself] Anna’s a lazy-bones.

The last three idioms are used of people outside the classroom situation too.