black diamond 
noun
1 informal A lump of coal.When first black diamond lumps were found beneath the ancient Appalachian ground a scene that none expected to see became as common as common can be....
- The first black diamonds were mined on ‘Coal Hill,’ now Mt. Washington in Pittsburgh, in the late 1700s.
2A dark, opaque form of diamond. Also called carbonado.‘He got her a black diamond with two gray diamonds on the side,’ says the source....
- Beautiful silver earrings hang from the points of her ears, encrusted by black diamonds and deep purple amethysts.
- Her black dress, shining from some silver silk material with black diamonds sewn into the hems and lines, contrasted with her washed-pale skin.
3 [usually as modifier] North American A difficult ski slope: a steep, black-diamond run...
- While I worry about getting on and off the chairlifts, however, my boyfriend, an experienced snowboarder, starts fantasising about soft, powdery snow, half-pipes and double black diamonds.
- Most Vermont resorts are best suited to intermediate skiers, although with 23 lifts and 145 runs - three of which are double black diamonds - there is more than enough variety to keep any skier busy for a week.
- You look at those tempting black diamonds, so colourfully named (Dog Face, Cascade, Paint Brush) and think, hey, they don't look so difficult.
- not be an option
- not bear thinking about
- not before time
- not be fussy
- not-being
- (not) be myself
- not be oneself
- (not) be ourselves
- not be someone's first rodeo (or not be someone's first time at the rodeo)
- not best pleased
- (not) be themselves
- not be worth a damn
- (not) be yourself
- not blink an eye
- not breathe a word
- not but what
- not by a long chalk
- (not) by a long shot
- not care (or give) a hang
- not care (or give) a hoot (or two hoots)
- not care (or give) a straw (or two straws)
- notch
- notch baby
- notchback
- notch bar test