While GIA is the gold standard for diamonds, a lot of people look to the American Gemological Labratory (AGL) when it comes to colored stones. You can get a simple GemBrief card which states a positive identification and any evidence of enhancement for $70. On the other end of the spectrum, they’ll do that plus origin and color grade in a Prestige Grading Report. The price for those depends on the carat weight of the stone, and can run you in the neighborhood of $400-1100.
So the cost is the biggest factor. Is it worth getting a stone evaluated for $70 if you’re only selling it for $150? Probably not. The only stone I’ve ever sent off was a 0.96ct unoiled emerald (less-than-awesome photo attached). We only got a GemBrief because the origin was nothing special (Brazil, but not Muzo), but for a stone like that, a simple report showing no evidence of treatment was definitely worth it.
–Eric