Hi, uncut gemstone are like any other gem, first you have to look at the quality as always the quality runs from A to Z, then you have carat weight, bigger the gem higher the price, shape of gemstone which will determine yield, market trend is the gemstone in vogue is it sought after more so than other gemstone. is it rare, ruby, sapphire, demantoid garnet, emerald, Tsavorite etc, at the end of the day there is no hard and fast rule you can apply to buying rough gemstone, except your knowledge and expertise you have gathered over a long period of time. I am talking 10 20 30 years here. and that you actually know what you are looking at in the first place
anyway just a bit of my thoughts on the matter, I am sure others on the forum have their ideas as well
thanks.