I've been here several times.  Most of the petrified wood found on the surface is 'crumbly' (if there's such a word)  the better pieces are on the hill side about 2 to 3 feet down.  They have a nice 'sing' when you strike them together... the lower quality pieces have more of a dull thud sound when striking together.

Be sure to stop for whatever supplies are needed first (Murphy, Bruneau, Black Sands, Grand View.  There are no service for miles once you're at the site) Access from SR 78.  At SR 51, go south to 'Last Frontier Road' (gravel road ... gps 42.770 -115.911)   From the turn off you'll clearly see some outcroppings and bluffs on up on the hill side.  Large patches of gypsum are mixed in the terrain.  Dive until you're above the bluffs and then there will be another gravel road crossing.  You can park there and walk or make that turn  if you're comfortable driving on it. (not a maintained road - possible deep ruts from water run off / erosion) From the gravel road crossing, the best finds will be about 100 yards in on that side gravel road to the south-west and then 50 to 100 feet up the on hill side to the east.