Describe a dagger?

The girl held out the belt. 

"Tie that around your waist. You never know who we might meet."

Lucy weighed it in her hand. "What is the heavy thing in the pocket?"

"It's a 'sheath', not a 'pocket'; and the 'heavy thing' is a 'dagger'.  Careful! You'll cut off a finger if you pull like that!" 

She stared at Lucy. You've never held a dagger before?"

"I...I don't know what a 'dagger' is."

"It's like a knife. You grip it here, like this." The girl grasped the [insert correct word for handle] of her own dagger.  "And pull it out like this." She withdrew the dagger from its sheath. The triangle of metal glinted in the sunlight.

"The blade is very sharp on both sides." She demonstrated by slashing the leaves on an overhanging tree branch. "The point is for stabbing your attacker. Very sharp. It will go into his chest like a knife into cheese." 

She stared into Lucy's widening eyes. "Yes.  You must kill him. I'll teach you how to do it "

Lucy gingerly pointed at the symbol engraved on the blade  "What does that mean?"

(I wished you had given us some background. Lucy has never handled a dagger, but why has she never seen one?  Most cultures have used knives or flints to stab, cut and saw through food and wood.  If Lucy has visited a museum today, she would have seen daggers, knives, and arrowheads. It would have helped me to know why she had never seen a dagger before.)

Someone who has never seen or held a dagger before is going to be of absolutely no use in a fight. Unless it's a magic dagger of course.

If you consider yourself to be "capable of writing", why are you constantly asking for assistance with simple things? How would I describe a sword or a gun or a golf club? It would depend entirely on which specific sword or gun or golf club I wanted to describe. They don't all look alike. 

It's a serpent that takes away her innocence to be replaced with thoughts of hate, gleaming in the sunlight like a miniature sword and every bit as destructive as the apple that Eve gave to Adam in the Garden of Eden. 

The demons carved into the handle represent the demons of sin and a passport to Hell.

-----------------------------------------------------------There, you don't need a detailed description. The idea is to show not tell. A drawn out description makes for boring reading so a suggestion of what it looks like is enough.  

To someone who has never seen a dagger, it would just look like a long knife with a sharp point.  One that probably has two cutting edges.