Young boy protagonist with a sword containing a red stone on its hilt? It was in a modern day world with guns and wars, there was some kind of armed conflict going on.

The story was about seven powerful crystals or gems or rounded stones, each carried by a different person wielding it in a way that suits his fighting:


One of them had it embedded in his shield, it was green I believe
One of the characters also carried his stone around and played with cards
The main protagonist had it in the hilt of his sword I think


In one of the scenes the boy was shown to have been born with the stone in his hand.

By the end of the series they were trying to unite the power of the stones to defeat some kind of enemy.
I think it could be Shin Hakkenden (1999, 26 episodes)


  The year is 2588 in the system of Godworld and its eight satellites. Humanity came here in the distant past, after a long journey across the stars from Earth. Now the system is ruled by a loose coalition of powerful families: House Owari, House Meed, and House Iigy. The story begins when Kai, the heir to House Owari, attacks an icy planet. Living in a humble village on the surface is Kou, a young boy who is perpetually arguing with his father. When Kai's navy arrives, Kou takes his father's mighty Murasame sword to fight back, and his adventures begin. A sci-fi remake of Hakkenden. (Plot from Animevice).


This is a picture of the main character Kou who has a red gem in his sword (You can also see a dog with an eyepatch: it hide a yellow gem in one of his teeth).



This is Jinrai who has a green gem in his shield.

Another pic http://gallery.animeinfo.ru/img/Jinrai-125835-57413-1.jpg

This is Tomoka who has a blue gem in a dice: he is a gambler and use cards also for fighting.