An Image of Gray

[Seraphim]

perhaps a twist of cruelty edging its aging lips,
edging aging lips, you seem to have 'its' for the meter,

For the rhythm.

I associate slab with marble, not diamonds. Also diamonds are cut not chiseled.

A slab can be made of anything - it's more, I think, a size, shape or dimension. So it's a slab made of diamond, not a finished gemstone. Diamonds are cut into gemstone. This one is not. Quite the opposite, I think. It's not sparkly and pretty. Its chiseled, yellow and flawed.
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Dorian Gray was eternally beautiful on the outside, but he was no polished gemstone on the inside.