In reading about "Jim's knife", all the accounts of the "original" indicate that it was a very plain looking knife, but that the sheath Jim carried was a "silver mounted black leather sheath", what-ever the heck "silver mounted" means.
There were a lot of accounts about what happened to the "original" Bowie knife, ranging from "Jim had it at the Alamo". to he took it back to the Bowie Plantation and gave it to his nieces/nephews, to he lost it/gave it away/had it modified/sold it, or my favorite, lost it in a poker game.
Rezin, on the other hand, was known for having knives made as "presentation knives" to be given away on various occasions.
My own personal theory, with zero documentation to back it up, is that Jim if he had not lost or misplaced the knife in some manner, then he had it with him at the Alamo.
A couple of things that my theory is based on--
1) His brother made it or had it made for him. There would have been a lot of sentimental attachment to the knife for that reason.
2) The knife was the one that made him "famous" at the Sandbar Fight and he would have kept it for bragging purposes.
My assumption is that since he probably had it at the Alamo, one of 3 things happened to it -
1) burned with him and the bodies of all the others killed at the Alamo
2) picked up by a Mexican soldier, who then carried it until he lost it, had it stolen, was killed, was captured at San Jacinto and had it taken from him, or took it back to Mexico with him and subsequently lost to history
3) flat lost in the dirt and debris of the destruction at the Alamo.
Overall, anytime someone says that "they" have Jim Bowie's Alamo knife, I just laugh and think "Yep, there's another sucker who's been separated from his money." Including the one that Phil Collins donated (along with a lot of other alleged "Alamo related artifacts").