Question: “Dear Grammar Guy, when is something valuable vs. invaluable? Are they interchangeable like flammable and inflammable? I’m hoping you can shed some light on the issue.”
Answer: Great question, reader. Thanks for asking.
I imagine what’s tripping you up here is that prefix “in-” – which sometimes makes things negative (i.e. “inoperable,” “incredulous,” “inadmissible,” etc.) and sometimes doesn’t (i.e. “inflammable,” “infamous,” etc.).
How we’ve come to have a prefix with different meanings is that English is a patchwork quilt of other languages. Latin, French, German and Greek, to name a few, all have visible influences on the language. Sometimes, as is the case with “inflammable,” that causes confusion. Latin has a prefix “in-” which means “not,” and a prefix “in-” which means “into” and acts as an intensifier. Thus you get the Latin “inflammare” – to set on fire – evolving into the English “inflammable” – able to be set on fire.
We have sort of a similar problem with “invaluable,” which is that it made perfect sense when it was coined in the 16th century, but not as much these days. 500 years or so ago, English speakers understood “valuable” to mean “capable of valuation,” or capable of having a price set on it. Thus, if something was “invaluable,” it could not have a price set on it.
From those origins, we get “valuable” today meaning something important or worth money, and “invaluable” meaning something that is “valuable beyond estimation” – often for sentimental reasons.
And so we return to your original question: Are “valuable” and “invaluable” interchangeable? The best answer is probably that, in colloquial speech, few people would stumble over your meaning if you used one or the other. In writing you should stick with using “valuable” to mean something with a quantifiable value (Like, say, a bar of gold) and “invaluable” to mean something that is valuable beyond measure (Maybe a family heirloom or a piece of artwork given to you by a friend).
A final note: “Invaluable” doesn’t always mean that something is literally beyond value. In the family heirloom example, you could get it appraised and sell it for some amount of money. Its invaluableness comes from the emotional importance you place in it.