It's just a stone... or is it? The Ebony Stone is an object of great importance, but Lament's plot and script provide little information. Walter keeps this precious artifact close at all times, set as the centerpiece of the necklace pictured to the left.
The Ebony Stone is an alchemical artifact of unknown origin. It existed for some time before the story of Lament; it covers the vampire's forest with an endless darkness, and Walter is described as having lived in the forest for so long that people have forgotten when he first arrived ("no one is sure since when"). Unfortunately, there is no way to know how Walter obtained the stone, so we are left to wonder if he created it himself, commissioned its creation, or stole it from a human or vampire alchemist.
The stone appears to be a simple black cabochon. Walter wears it in a heavy gold necklace, keeping the precious artifact close at all times.
It seems that the stone's only "visible" effect is to maintain the eternal night that surrounds Walter's forest, but this conclusion is incomplete. It's difficult to puzzle out the power of the stone, as the script doesn't contain much information about it. Most of what we learn is filtered through Joachim, who isn't exactly the most sane person in the castle, or really motivated to speak truth to Leon. Let's look anyway:
Joachim: Shut up! Damn Walter... I would have won, were it not for his Ebony Stone.
Leon: Ebony Stone? What's that?
Joachim: If you want to know, beat me. It'll be a nice prelude to defeating Walter. I'll beat you to a pulp.
Leon: You promised, answer me. What is the Ebony Stone?
Joachim: A stone made with alchemy... It creates the neverending darkness in this forest.
Leon: Hence, Eternal Night... But the night should help you as well, since you're a vampire.
Joachim: That stone chooses its master... My powers pale beside his. Oh, if only I had the Crimson Stone...
From this, we can infer two things (assuming Joachim isn't totally crazy, which isn't really a safe bet, but it's the best one we're going to get here...):
First, the stone enhances a vampire's power. Joachim boldly declares that he would have won the fight with Walter if not for the stone, and is rather huffy about it, almost as if Walter cheated somehow by having the stone. After he is defeated by Leon, Joachim adds that "my powers pale beside his," a statement that makes little sense considering how bold he was earlier. It's obvious that the stone has made Walter's powers stronger.
Second, the darkness created by the stone only benefits its master. Joachim was not protected against Leon's incomplete weapon (or anyone else who happened by his room) by the endless night; he had to build his own defense system by drawing on the power of his swords.
Obvious stuff, to be sure, but the script is painfully thin on detail here.
Black, like other colors, has a laundry list of symbolic meanings and superstitions attached to its usage. One that comes immediately to mind is the association of black with nighttime and the evils thought to be hidden by the darkness. We have expressions like "black heart" and "black magic," both suggesting deception or evil. Black is also a traditional color of mourning in the Western world.
On the other hand, black can be a powerful positive. It is a color of elegance and class as seen in the idea of a "black tie affair" or the "little black dress." We say that "[xyz] is the new black" when referring to an essential element of fashion, and believe that black is a slimming color. In the world of business, black represents profit; consider Black Friday, so named because sales on that day supposedly push retailers from loss to profit, or "into the black."
Black can also be a symbol of power, sexuality, mystery, emptiness, or anger. It seems like an endless conflict of ideas, but this variety allows color to be used more freely and with subtlety of meaning instead of remaining a simplistic and stereotyped symbol. In the case of the Ebony Stone, it's reasonable to conclude that its color is meant to symbolize the power and sensuality of a vampire, the evil of unnatural darkness, the mystery of alchemy, and the elegance of its "stubbornly refined" owner.
While it's obvious that the word ebony is used to describe the color of the stone and not its composition, it's interesting to note that if the Ebony Stone were actually made of ebony, it wouldn't be a stone at all. Ebony is a strong, dense black wood that has been used for ornamental purposes for centuries. It has been used for chess pieces, instrument keys, ornamental carvings, and even furniture. Items made of ebony are less common now due to the unfortunate overharvesting of the trees for their wood.