Ileney; "IMO". Well, Gem Grade seems to vary by the person although it is defined. Most "gem grade" rock collectors who buy from me have simple rules: No pits or tiny fractures anywhere on the slab. Rough rock you have to guess a bit. Is there any crazing? So an end polish shows at least a bit and it should take a polish like a show car with 10 coats of paint. There is a lot of information on the internet from respectable sources such as the GIA on how to judge gemstones. Many of the rules apply to jasper that apply to gemstones. If there is any type of treatment then it is not likely to be considered Top Gem. Turquoise gets an exemption from many people but "Top Gem" means no treatments of any type if you are going by the book such as GIA.
There are still a dozen other variables that affect value. Color in coral is a big deal. Some pinks and the deepest reds are hugely collectible. The diameter and total size of the rough is also a big factor. Is it pretty, does it take your breath away, did you reach for your wallet before you asked the price?????
I also sell collector grade material that is expensive but does not always meet gem grade. Much of my finest Fantasy Plume Agate has pits and has small internal cracks. But the stone has such an incredible image, a superb gloss finish and the shape fits the image, that it is still a collector cabochon. One of my finest pieces of plume agate was in Lapidary Journal a couple years ago. Since I mined the material myself, cut the cabochon and did the silverwork I was a bit pleased. There was also 12 pages of my Deer Sky Picture Jasper in that issue.
MYTHS: Old stock is always valuable. Not if it is butt ugly.
Bigger is better. Not if you cannot get it into your saw or afford to ship it.
Rare is valuable. Again, if it ain't pretty most people do not want it.
I know this is Owyhee Jasper because XXXX told me it was. He would never lie and knows more than anyone else about rocks.
Always take a deep breath when dealing with rocks and keep a good sense of humor. I am still learning and this is just my opinion. But it is based on many years as a miner, seller and cutter of agates and jaspers.