Yes -- horn can be dyed.
plastic buttons are all identical, color, pattern, grain, shape. they are all identical.
although some manufactures can vary the color pattern.
horn all are different. check the grain pattern it should vary. sometimes you can feel the grain especially on the back side. and the color will vary a bit.
plastics are made by injection in multiple molds.
horn is made from blanks stamped from the horn, then turned one at a time on a lathe. then stained and polished.
any wonder they cost so much more.
I have never seen a high-end RTW jacket with anything but plastic buttons, by the way.
This is going to sound weird...
Well, not completely, but feel the button against your teeth. No matter how well polished, I find horn feels a bit "grittier" and "firmer" than plastic (which seems slicker and more brittle).
Told you it was weird. But it also works for discerning mother of pearl from plastic too.
this is good, but many people have a hard time with the tooth method. If you have access to magnification (10x or more) you may be able to see an "engine turned" effect. If you can see this, it is definately real horn. If you cannot however, unfortunately it doesn't definately mean that it isn't.
ref: Gemological institute of america
I took my wife to see the buttons. She observed that the holes for the thread are a different color inside, which she, I think correctly, believes rules out plastic.
How about composites, that are made of horn dust combined with synthetics? I think I recall reading about those.