Although I agree with the others that telling your mom the truth was the right thing to do, I can also understand your desire to delay telling until you had a replacement ornament in your hand.
Having the replacement would have "softened the blow" of the bad news that the original one was broken beyond recall. And that, my friend, was a very generous thought for you to have.
Have you ever read the short story "The Gift of The Magi" by O. Henry yet?
Hollywood of course made a film version of his original tale, but the story is a better read.
In brief, it went like this:
A husband and wife were very, very poor. They each only had one thing of value. He owned a beautiful, heirloom pocket watch and she had marvelous, knee length hair.
They had no money for Christmas, yet each wanted to give the other something wonderful.
She saw a watch fob in a store window that would be perfect for his pocket watch which didn't have a fob.
He saw a beautiful set of hair combs, the kind that hold long hair in place when worn in an upsweep.
But how to afford these expensive gifts?
She cut her hair and sold it to buy the watch fob for him.
He sold his watch to buy the combs for her.
It's quite a story, isn't it? Oh, and MUCH better as told by O. Henry!
But you see, I recognize something of their generosity in what you were trying to do for your mom by wanting to hold back from telling her about the broken ornament until you had another one.
What wouldn't a kid try to do for his mom at Christmas?
So although you did the right thing by telling her immediately (it is always better NOT to lie to a mom!), I think you can also be proud of yourself for considering the idea of waiting to tell her (to be kind and generous).
You are a great kid, Mark, and I'm sure your mom is proud!