Crescent Rose

Crescent Rose is Ruby's weapon of choice.

Description
Crescent Rose is a High Caliber Sniper-Scythe (HCSS), a hybrid between a scythe and a rifle and, according to Professor Ozpin, one of the most dangerous weapon types designed. It has very high recoil, which Ruby uses to great effect in her mobility; firing shots to propel her high in the air or quickly at her enemies. In order to use Crescent Rose to its fullest potential in melee combat, Ruby can fire a shot immediately before a swing, using the recoil to accelerate the scythe, increasing the power of her attacks.

Crescent Rose can fold up tightly for easy transportation and for convenience, stored underneath Ruby's cloak, at her waist.

Although Crescent Rose can fold into the form of a gun, which bears a small resemblance to a shotgun, it does not have to be in that form to fire. It appears to be a bolt-action weapon, as Ruby can also embed the scythe's blade into the ground to prevent the recoil caused by firing from sending her flying.

Construction
Crescent Rose appears to be a highly advanced technological piece of equipment, being able to freely transform from its compacted self, to its gun mode, to a full scythe quickly. As its name implies, its color is rose-red with black trims and the blade is crescent in shape. In Episode 2, Ruby comments that she was the one who designed the Crescent Rose as students of Signal forge their own weapons, claiming she went overboard with it's design.

The weapon appears to have two separate types of ammunition. When Ruby reloads, she can be seen inserting a magazine with a cross (4 arrows pointing towards a central dot) etched on it. Her subsequent attacks and movements are significantly more powerful than those with the “regular” ammunition, likely due to the shots themselves being much more powerful, increasing her speed through the increased recoil of each shot. Monty hinted in a tweet that the RWBY community had missed something in the "Red" Trailer, and followed by posting a picture of Ruby during her last shot. It is unknown what the hint may have referred to, but the enhanced bullets may create a cross-shaped blotch whenever one is fired.

Interestingly, the Crescent Rose is capable of bending its blade backwards. This form largely resembles the traditional war-scythe, a combat modified version of the old scythes.

Trivia

 * Some fans have been known to call this the "scythle" or "rythe" (as in, "writhe in pain") as a callback to when Red vs. Blue's Agent Maine first obtained his Brute Shot, which one soldier commented upon seeing it "Is-is that a knife? A rifle? Kn-Knifle?"
 * The Scythe has a small, jagged blade at the bottom of the shaft, perhaps used for her swirling attacks.
 * Ruby seems to have no difficulties with 'unsheathing' it or extending the scythe's blade, suggesting that Crescent Rose is mechanical and can operate itself.
 * Professor Ozpin notes that Crescent Rose is "one of the most dangerous weapons ever designed." However because Ruby claims to have designed it herself, Ozpin is most likely referring to scythes in general. Another explanation is he examined Crescent Rose himself, since it is not present on Ruby during her interrogation.
 * It is likely that Ruby partly named the weapon after herself, since it bears her last name. Crescent comes from the shape of the blade, the same as a crescent moon.
 * Ruby at one point affectionately referred to the weapon as Sweetheart, showing her fondness for the weapon.