Usually a vague appraisal is bad for the consumer. The insurance company has some leeway with what quality diamond they can replace it with.
Since the quality is not nailed "dead-on" (like for example E color, VVS2 clarity, with a 1A make), the insurance company can replace it with what their experts consider "very fine" color, clarity, and cut (for example I color, VS2 clarity, with a 3A make). The difference in value between these two stones can be considerable.
In all likelihood, you probably had a pretty nice stone. If the appraiser really knew what "very fine" was, it was probably G color or better, VS2 or better.
Just out of curiosity, how much did it appraise for 25 years ago, and what was the exact date of the appraisal? There was a tremendous "spike" in diamond prices from late 1979 into early 1980. Hopefully your appraisal missed it. If it did, try this. Take the original figure, and multiply it with an annual increase of 5% from the date of appraisal till now, and post both figures. Maybe we can get a "feel" for it that way.
(If it happened to be appraised in January of 1980, cut the price in half, then shred it in thirds, and then stomp on it a bit. That will put you in the current neighborhood of price compared to that insane month in history)