What to do with a specimen that has no data
Updated
Every collection contains some. The question is what to write, and the answer is never a guess that reads as a fact.
Do not invent
The temptation is to assign the likely locality: it looks like the material from a known mine, so it probably is.
Write that down as fact and you have manufactured data that will outlive you and be repeated. It is a damaging thing a collector can do with a pen.
The honest label says no locality, and that is a complete and respectable label.
What you can honestly record
Where you got it. A dealer's name, a show, an estate, a box bought at auction. That is real provenance even when the geographic locality is lost.
When you got it.
A qualified opinion, marked as one. "Resembles material from X; no locality data" is honest and useful. So is "said to be from X, unverified".
The qualifier is what separates a note from a claim, and it must survive onto the label rather than living in your head.
Selling it
Sell it as what it is: an attractive specimen with no locality. Buyers accept that readily and price it accordingly.
What buyers do not forgive is discovering that a confident locality was a guess. That is a reputation problem in a small world where dealers and collectors talk to each other.
The one exception
Self-collected material where you remember the trip but not the exact spot. Write what you do know, the trip, the date, the general area, because that is genuine first-hand information, and it is far better than nothing.
Then go back with a notebook and do it properly next time, which is the actual lesson of every undocumented drawer.