Most of these blocks are cut from a strange mutation that can happen with Redwoods underground. In the roots a growth that is shaped like a big zucchini can happen. This is not a true burl but can have similar eyes and figure. The wood is a lot darker because of the minerals absorbed from the ground. I don't know if you would call this rare, but is is not a normal occurance.
In the photos I placed 2 normalish pieces of Redwood to give a color comparison.
The 1st photo is under florescent lighting without flash. The wood looks kind of chocolate brown.
The 2nd photo is with a flash showing how when the light hits it from an angle, orange and gold colors appear.
I will put these blocks in our webtore pretty soon.