Caelum

Caelum the "Chisel" is a Celestial Spirit that can transform between three things for its master Angel. Apparently it is only an item and not a sentient being.

Oracion Seis arc
Caelum is summoned to do battle against Lucy Heartfilia. As it is summoned, it fires a blast at Aries and Loke despite Aries being on its side. Gemini then uses Caelum in sword form to attack Lucy but as Gemini refuses to hurt Lucy, Angel takes Caelum to attack herself. In the end though, Caelum is defeated by Lucy.

Forms
1. Cannon Form: The first form to be used and a therefore a good candidate for its default form. In this form it appears as a high tech cannon that can launch powerful blasts of energy. Firing Caelum might require focus or stamina from it's summoner, as Angel was shown to be too injured and beaten up to think straight after getting hit by Uranometria, and the energy beam she fired curved around Lucy and hit the raft Natsu was on instead.

2. Flight Form: The second form to be used. It is a another good candidate for Caelum's default form because in this form it appears as a simple winged sphere which seems to make it the basis for the other forms. This form has yet to be seen using any attacks and it probably has none. Though it's likely this is the only form shown that can move considering it's the only one that looks like it could, but it hasn't been seen doing this yet, so there is no proof of this.

3. Sword Form: This form has the "chisel" part of the sphere enlarged so much it looks like the Chisel is the blade of a sword with the Sphere part being it's hilt. This form is in fact the first evidence of some stellar spirits having no ability to resist their masters and it seems Caelum even lacks a personality or speech abilities which makes it very different from other Celestial Spirits shown so far.

Trivia

 * "The Chisel" is the modern translation of the constellation Caelum, but apparently that's not quite accurate. The word is ambiguous, in Latin caelumn means both "the heavens" and "burin," which is an instrument used for engraving on copper and fine metals. It is this instrument that Lacaille had in mind when he named the constellation. If Hiro Mashima intended the name of Caelum to be "The Chisel," the correct name would be scalprum, which means chisel in Latin.
 * It is the the only Celestial Spirit Lucy didn't get after the Oracion Seis Arc.