Reading Walnut

When I go to the walnut mills looking for gunstock wood, I particularly look for wood that has good grain flow, contrasting color, dark mineral streaks, or interesting figure.  I have long experience reading walnut, so I can spot the good stuff in the rough cut and dirty state I find them in.  This allows me to provide walnut at a grade or two or three better for the price.  I may literally look through hundreds of blanks at the mill and only find a dozen interesting ones.  I encourage you to go look at walnut gunstock blanks online.  There is a ton of expensive plain wood out there, and astronomically priced pretty stuff.  You will find the wood I have to offer is more interesting than other walnut blanks at each price point because they represent the top 10% of wood I look at.  
When looking at gunstock blanks, there are a number of different elements to consider to be an effective judge of the relative quality, and value, of the blank you are looking at.
Gunstock blanks have several features to consider.  Those features have to do with aesthetic features and strength features.  Few gunstock blanks that come out of a walnut mill fit all the criteria, and those are very expensive.  Understanding the features gunstock wood demonstrates will better help you recognize the value of a piece you might be looking at.  I've seen plenty of nice walnut gunstocks on airguns split across the wrist because the stock was laid out upside down on the blank or the blank was not suitable to begin with.  After reading this information, you'll be much better informed to evaluate guns and wood you may be looking to buy.
Features of strength has to do with the strength of the wood itself, and the direction of the grain flow.  Typically we look for wood that has the grain flow up through the pistol grip and wrist and also sweeps slightly up through the forearm.  Straight grain blanks are the least expensive, but gunstocks made from straight grain wood can shear at the wrist.  If you look at the wood grain going straight across the thinnest part of the grip area of a gunstock and it goes basically straight across, you can see how the wood can break.  If the grain flows up through the pistol grip, it is very strong.
Sap wood can impart an interesting look, but it is important to understand that sap wood in the blank is generally considered a price detractor.  For airguns, a little bit of sap wood is just fine if the wood otherwise has some interesting figure or color, and then we don't have the recoil of a .300 WinMag.  Some sap wood in a blank can help an attractive piece to be attractively priced.  Sometimes the sap wood will be outside of the gunstock, but then that does not tend to lower the acquistion cost.
Runout: Another component of strength is the potential for warpage during carving caused by grain runout.  Grain runout is another consideration that has pluses and minuses. By runout I mean if you look down at the top and bottom of the blank rather than the side, does the grain flow straight down the blank, or does it curve out the side (runout).  Virtually every blank will have some small level of runout, but excessive runout can be a problem, yet can also create stunning patterns. Runout means the stock should be carved 1/2 way and let it finish moving, if it does move, before finishing the carving.  Such a blank will finish straight, and runout can create very interesting patterns on the side. Reading runout in a blank is very challenging because it is the roughest cut on a blank, but when identified as such, either through recognition of the pattern on the side or by being able to visually see the runout through the rough cut on the top and bottom, can keep the price down on an otherwise great piece of wood.  But some blanks may be made more spectacular by the feature, so there's not necessarily a price reduction, maybe the opposite.  An educated carver will recognise blanks with this feature and treat them accordingly.
Aesthetic features have to do with the flaws of nature and the magical appearance of the wood.  Traditionally a consideration of value deduction is small knots from tiny branches, fissures, and voids, and while a 'perfect' blank with high figure, perfect grain structure, and no tiny knots or holes is the holy grail, they are also expensive. Holes or little 'birds eyes' twigs don't bother me. If a stock meets all the other requirements and is beautiful, but comes in at an attractive price point because of small knots in the figure, that's perfect for us airgunners. I epoxy fill those voids and they completely disappear.  I look for details like that when sorting wood, and as with sap wood, a blank with knots in it had better have some very redeaming qualities, but often when the wood does have those other qualities, they so completely drown out the odd little birdseyes in the figure that the eye doesn't notice.  However, such a piece can have an attractive price point for an otherwise spectacular blank.
Figure is of course a prime consideration. Northern California Black Walnut (Claro) which is the wood I'm surround by, is world renown for it's gem like quality appearance. The 3d rippling wrinkled wood look, the reds, yellows, greens, and browns of contrasting color, and the potential for dark swirling mineral streaks almost as dramatic as English make for heirloom quality gunstocks. It is a challenge to see figure and color in a blank in it's rough state. Figure that will amaze you and 'pop' when the grain is filled with oil and polished is virtually invisible in it's rough hewn state at the mill. This is where the experience of reading wood comes in the most, especially when sorting junk after junk at fairly high speed but spot the gem through a rought bandsaw cut in a moment. 
Considerations of figure include: there might be significant figure on one side of the blank, but none on the other. That means the figure is falling off as it passes width-wise through the blank. Will there be any left on the good side after the stock is carved down inside the blank? Is that figure showing on the near side or off side? Blanks with figure on only one side are much less expensive than a blank that is fully figured from one end to the other equally on both sides. More than that, a blank with proper grain flow AND fully figured become quite expensive. and few airgunners are willing to pay the price of a Daystate for a chunk of wood. 
Color refers to contrasting colors.  Dark mineral streaks or contrasting streaks of wood or dark and light areas all create the appearance of contrasting colors.  Nice color can make a beautiful stock even if it doesn't have figure per. se. 
Exhibition Grade: Any one of the above elements in strength and or appearance adds value.  The best wood would emobdy all of them with minimal runout, have the grain flow up through the pistol grip and slightly up down the length of the forearm, have nice contrasting colors, and be overlaid by a rippling pattern of figure.