Why you can’t buy furnitures: The ins and outs of count and noncount nouns, The American Heritage Dictionary. From Steven Pinker. twitter 
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Furniture is an example of a noncount (or mass) noun—it refers to a mass quantity, rather than to a discrete item, and it can’t be pluralized.
Chair is a count noun: a chair is a single thing, and it can be made plural by adding the letter s.
Many nouns with two forms have a noncount sense for an abstract meaning and a count sense for a specific occurrence.
Conversation in the abstract sense: “He is gifted in the art of conversation”
Conversation in the count sense: “He held a long conversation on the subject”
Careful analysis of usage is often required to determine how a particular noun is used, and count or noncount usage can shift.
A phrase like piece of is the standard way to to refer to just a bit of a noncount noun. Many noncount nouns have their own particular phrases that usually go with them.
a gust of wind
a grain of rice
a drop of rain
a cup of coffee
a stick of gum
a bolt of lightning
a bit of luck
Sometimes we leave out the introductory phrase, turning the noncount sense into a count noun. No one will blink if you order “a coffee” instead of “a cup of coffee.”
For additional journal history, see Can't Buy Furnatures from where this content was ported.