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Q and A: Could My Investigator Determine If the Knife Used in a Murder was Made of Obsidian Rather Than Some Other Material?

26 Mar

Q: I am a lawyer in Tacoma, WA with a hobby of writing detective novels, all unpublished but fun for me. My question is would a knife wound from an obsidian knife be identifiable as from an obsidian knife as opposed to a knife made from another material?

John C. Cain, Tacoma, WA

A: Not likely, unless the knife had an unusual shape or curve or both. Wound analysis will only give the width, thickness, and general shape of the blade as well as its minimum length. The depth of the wound would tell the ME what length of the blade entered the victim. The blade could be longer but not shorter–thus the minimum length. If the ME then had the suspected murder weapon he could measure it and say that this blade was or was not consistent with the victim’s wound. That’s as far as he could go. He could completely exclude the knife as the murder weapon if the wound didn’t match but he could not say that this blade, and no other, made the wound. Only that this blade or one similar to it did the deed. The more unique the blade is the better this would narrow the possibilities.

 
But there are a couple of ways he could make a more conclusive judgement. If the victim’s blood was found on the weapon, say in the groove between the handle and the blade where the killer would overlook it and where even washing the knife might not remove it all, he could then DNA match this to the victim and say that this knife held the victim’s blood and was very likely the murder weapon. Why else would the blood be there?

 
Even better, if the point of the knife broke off in the victim and this was found at autopsy, then the ME would know the knife was made of obsidian. A comparison of this tip with the suspect weapon could prove very conclusive. If the tip fit the suspect blade in an exact jigsaw fashion this is very conclusive and individualizing evidence. That is, the ME could confidently say that this tip came from this knife to the exclusion of all others. The science behind this is that no two things fracture exactly the same way.

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5 responses to “Q and A: Could My Investigator Determine If the Knife Used in a Murder was Made of Obsidian Rather Than Some Other Material?

  1. Mark A. Lewis

    March 26, 2012 at 1:55 pm

    The thing about obsidian is that the edge is extremely fragile, so, more likely than not, the would be particles of obsidian in the wound track.

     
    • D.P. Lyle, MD

      March 26, 2012 at 7:17 pm

      And thus excellent forensic evidence. Thanks.

       
  2. John C Cain

    March 26, 2012 at 7:29 pm

    Do you think the knife could pass undtectied through most courthouse security checks. My thought was tht the murder would occur in a courthouse.

     
    • D.P. Lyle, MD

      March 26, 2012 at 7:32 pm

      Maybe. It would show up on X-ray but not a densely as metal would and it could have an odd shape and go unrecognized and slip through.

       
  3. jill

    March 10, 2013 at 1:43 pm

    Don Crabtree did many hands on experiments with obsidian and other stone tools, they make a better cut than metal. If you look at eccentric flints you can see they can be almost any shape.

     

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