I read this back in the 90s. The protagonist ends up getting a gem embedded in his chest. Afterwards he had all sorts of extra strength and was all epic and stuff. But, come to find out, the woman who was tagging along was actually an alien and had embedded the gem inside him to test if he 'was a good fit' and ends up betraying the protagonist and he ends up as the space ship's power source, or the computer, or navigator or some such ending?
I vaguely remember the cover of the book was one of those "hey, look at how STRONG and AMAZING I am" power fantasy covers, but the book ends with the guy being betrayed and regretting his decision to get the gem placed inside his chest for more power. Initially he just carried the gem around with him, I believe; the woman convinced him to embed it into his chest.