Valuing Corundum
How much corundum is worth varies significantly based on its quality, potentially becoming anything from sandpaper to sapphires. Most corundum is opaque, muddy colored, and highly included. This material is industrial grade, getting used in things like abrasives, sandpaper, and power-tools. Industrial grade material is not even worth $1 a carat, especially with how cheaply other synthetic materials with similar hardness can be made.
Corundum that grows very similarly to its habit is valuable to specimen collectors, even if the colors are not particularly spectacular. Most of the time it looks very lumpy, and identifying the habit can be impossible in certain cases (like being a water-worn pebble). Mineral specimens that show ideal color and shape can go for millions at auctions depending on the quality.
Quality Grades
There are different varieties of higher quality corundum. They are mainly grouped into sapphires and rubies, along with a shared set of grades. The lowest is highly-included to opaque carving-grade material, cabochan-grade material, and lastly facet-grade material. Amongst facet-grade material, only the clearest pieces are made into emerald cuts.
The maximum possible value for a piece of corundum would be found in facet-grade rubies. These rubies have exceeded over a million dollars a carat, and are always used in jewelry.
Carving Grade
R2005
Cabochon Grade
R2092
Facet Grade
R11951
Finding Gem-Quality Rubies
Ideal crystals for faceting will not always present themselves as such. Gem cutters do take losses on uncut crystals that may or may not have good-quality material inside. However, there are ruby crystals that present themselves readily as high-quality gems and low-quality ones. There is a tremendous amount of value in gems that readily tell the cutters what they can make from them, rather than having to gamble at the answer.
Some of these pebbles might be able to qualify as a ruby after being cut, though most only seem to be viable as pink sapphires.
All of these crystals qualify as rubies even if they are pinkish. The gem rough that displays perfect reds and transparency fetch the highest prices and often sell immediately.
Desirable Features in the Rough
The most desirable qualities in faceted rubies, primarily color and clarity, are also the features most desirable in uncut rubies. These are the factors nature alone decides, since nice color and clarity cannot be manufactured in natural rubies. Clarity can be especially important since highly-included gems can also have durability issues, and possibly shatter as they are cut on a grinding wheel.
Larger gemstone rough is desirable, since bigger rubies are rarer and more valuable. However, it depends on how much is usable to be faceted. As mentioned above; choosing crystals to facet can be something of a gamble, even for more experienced gem cutters.
From there the cutter determines the best way to orient the gem for the inclusions and color, and will go with the option that will yield the most valuable ruby possible.