like chalk and cheese

Though attested since the late fourteenth century and surely encountered in every day speech, we were unaware of this delightful idiom, said of things that are superficially alike but very different in substance, like a crumbly, unaged cheese that’s never mistaken as flaking chalk (though some attribute the etymology to an unscrupulous cheesemonger that tried to pass off adulterated product). The Turkish equivalent Dağlar kadar farklı, “As different as the mountains” conveys the same sense. Its extended meaning covers things that don’t pair well.  Learn more at Nag on the Lake at the link up top.