The unusual wood grain you ask about is known as 'spalting' or 'spalted. It is a natural variant, caused I think by fungi [but I could be wrong], sometimes going to punk, but not always by any means. It is often very desirable and is used for wood turning of bowls that are sold as art work because of their beauty and the difficulty of turning it without breaking it. I do not think it is often found on a gunstock and I would treasure it accordingly.