game of thrones - Why did the show use a fancy looking Valyrian steel dagger?

This question points out that the dagger in the books was actually meant to be plain:

The blade Joff chose was nice and plain. No goldwork, no jewels in the hilt, no silver inlay on the blade. King Robert never wore it, had likely forgotten he owned it. Yet the Valyrian steel was deadly sharp . . . sharp enough to slice through skin, flesh, and muscle in one quick stroke. I am no stranger to Valyrian steel. But he had been, hadn't he? Else he would never have been so foolish as to pick Littlefinger's knife.
_{A Storm of Swords, Tyrion VIII}

However, in the show the blade is obviously fancy and goes against this description.

In the paragraph above Tyrion describes Joff's own dagger:

The prince's own dagger had a jeweled pommel and inlaid goldwork on the blade, Tyrion seemed to recall. At least Joff had not been stupid enough to use that. Instead he went poking among his father's weapons. Robert Baratheon was a man of careless generosity, and would have given his son any dagger he wanted . . . but Tyrion guessed that the boy had just taken it. Robert had come to Winterfell with a long tail of knights and retainers, a huge wheelhouse, and a baggage train. No doubt some diligent servant had made certain that the king's weapons went with him, in case he should desire any of them.
_{A Storm of Swords, Tyrion VIII}

Note that GRRM also thinks the dagger is different in the show:

A handsome blade, I think. Though rather completely different from the dagger in the books, which was far plainer (though made of Valryian steel, with a dragonbone grip).

Did the show intend for the Catspaw dagger to actually be Joffery's or did they make a mistake in the props department?

The question is essentially: why was the dagger in the show fancy when in the books it is plain?

Answer

I would imagine that it was simply to distinguish it from other, regular daggers.

The Valyrian Steel dagger is referenced many times in the first book/ season, and is the catalyst for almost every major political thing that happens until Ned's death.

It is used when attempting to assassinate Bran, it is used as the justification for Catelyn arresting Tyrion, Littlefinger puts it to Ned's throat just as he betrays him.

In the books, it is easy to describe it as a "Valyrian Steel" dagger, which immediately makes it special without us having to see it, yet it being plain allows for the excuse of Joffrey selecting it to order Bran's death.

But for these occurrences to have as much significance in the show, it really can't appear as a dagger that is just a regular dagger unless inspected closely. Otherwise it would be much harder to justify to the audience how it is so recognizable, or have as much of an emotional impact for each of these moments.

If I remember correctly (though I might be wrong), the issue of Tyrion tracing it back to Joffrey is not done in the same way in the show as it was in the book, i.e. realizing that it was Robert's knife and assuming that Joffrey chose it mistaking it for a normal weapon.