This story had a somewhat interesting concept, with the idea of a mark of the diamond that killed. I do think that this story is very original. I advise the author of the story to write with more description, like "as the blood slowly crept across the floor..." to make the story more intresting. Also, if this is a horror story, then I say, you have failed in making me scared at all. (And I am a very easily frightened person.) Like I said, write with more description so it'll be easier for the reader to invison it. On another point, the story is lacking any figurative speech. I'd put some smilies, if I were you.
But in all, this story wasn't so bad. Keep on writing!
-(I'm a 12 year old, just like you, unnamed author. So no excuses. And don't tell me I sound like a teacher.)