From New Oxford American:
dark horse |ˈdɑrk ˈˈhɔrs|
noun
1 a person about whom little is known, esp. someone whose abilities and potential for success are concealed : [as adj.] a dark-horse candidate.
2 a competitor or candidate who has little chance of winning, or who wins against expectations : a preseason dark horse as the nation's top collegiate football team.
sleeper |ˈslipər|
noun
3 a movie, book, play, etc., that achieves sudden unexpected success after initially attracting little attention, typically one that proves popular without much promotion or expenditure.
• an antique whose true value goes unrecognized for some time.
From Etymonline:
dark horse
in politics, 1842, an image from horse racing, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
sleeper
[...] Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in Amer.Eng,. sports jargon, probably from earlier gambling slang (1856) sense of "unexpected winning card."
Here's some generalizations gleaned from the above:
- dark horses are usually people; sleepers are usually things.
- dark horses are intentionally kept unknown; sleepers are simply not promoted.
- a dark horse might achieve success; a sleeper already has—surprisingly.
- dark horses' limelight often decrescendoes; sleepers' crescendoes.
As to your questions, I'd say they're not quite interchangeable. Knowing their different connotations will likely prove one to be the better word choice.
A dark horse might also be called an enigma, or simply an unknown; while surprise hit, cult classic, and box-office success are other phrases often applied to sleepers.
For the most part, the words are interchangeable. Distinguishing between multiple examples of such things can be aided by their individual connotations:
crack
a line on the surface of something along which it has split without breaking into separate parts
A crack tends to be a visible flaw that can splinter or spider into larger cracks with many smaller, attached cracks. The defining point of a crack is that the cracked object is still together — no matter how tenuous. Often there will not be any visible negative space or hole. Most cracks can be felt by rubbing your finger or hand over it.
slit
a long, narrow cut or opening
The biggest difference between a slit and a crack is that a slit implies some form of opening. Cracks also tend to be on a surface; a slit can be in virtually anything. A stereotypical slit is one that causes some portion of the object to pull apart — large slits can go all the way through to reveal a clean hole.
crevice
a narrow opening or fissure, esp. in a rock or wall
A crevice is typically reserved for very large objects that have been separated into more than one distinct section. A crevice in the earth would be something that separates two plains; a crevice in a wall could very well separate the wall into two pieces. "Narrow" is respective to the larger object. A crevice could be meters wide or only inches. A typical crevice is also very deep.
split
a tear, crack, or fissure in something, esp. down the middle or along the grain
A split more directly conveys an object being split into smaller pieces or a longer top-to-bottom crack. A split could also be used to describe pieces that are no longer attached at all. There is no implication of depth with a split; the importance is the length of the split or how much of the surface remains unaffected.
cleft
a fissure or split, esp. one in rock or the ground
When I think of a cleft I typically envision a V-shaped hole. A "cleft in the rock" is a V cut sideways into a mountain that can provide shelter. The "cleft of the chin" is the middle gap in some people's chins. Depth is important to a cleft but the gap will not go all the way through.
To directly address your example of a flaw in the bottom of a bowl, I would expect crack to be the most applicable. If there is a chunk missing but no thin lines it could be best described by chip:
a small piece of something removed in the course of chopping, cutting, or breaking something, esp. a hard material such as wood or stone
Best Answer
To answer number 1: the difference between "expertise" and "specialty" is that you can have expertise in many areas. Your specialty is your primary area of focus.
Regarding number 2: I'd recommend "expertise" as it allows you to enumerate the various areas in which you have skill. In fact, I'm not sure I've ever seen "specialty" used in a CV. It's certainly uncommon. But "expertise" is quite usual.
Also, the term "speciality" is preferred UK usage, whereas US standard is "specialty."