A Bone-Mark is an ancient artifact, generally containing the spirits of ancient war-dragons from 3000 years ago. Some also hold the spirits of dragons who lead the different empires, but some like The Dragon League of Eight hold spirits of elite dragons rather than empire leaders. Other bone-marks contain the spirits of certain Booster's dragons from ancient times as discussed in Episode 0: The Alchemist.
Bone-Marks also hold a lot of pure draconium energy, making them very powerful, and are constructed of pure crystalline draconium. These Bone-Marks can attach themselves to modern dragons and possess them if the dragon is not strong or pure enough to absorb the mark. The dragon will remain possessed by the spirit until the Bone-Mark is then removed in some way, if not absorbed. If the dragon is of pure draconium, the mark can be fought and absorbed, and the dragon can then call upon this ancient spirit and power, usually transforming them into an alter-ego of themselves whilst still in control, and gaining immense powers as a result.
Cyrano was transformed by the Samurox's Blue Draconium Bone-Mark but because Cyrano is of Green Draconium, the Bone-Mark was not fully attached and was eventually removed from Cyrano, who proceeded to drop the Bone-Mark into the deeper depths of the city. From this point on, the Bone-Mark was never seen again.
Beau was also subjected to the Furox's Red Draconium Bone-Mark and after wrecking much havoc, managed to absorb it into himself. We later see in other episodes where he calls upon this power in times of need, transforming himself into a Furox Form.
Decepshun was given and absorbed the Vysox's Black Draconium Bone-Mark, and this alter-ego of herself is called upon whenever Moordryd Paynn becomes the Shadow Booster. It was also noted in the series that after absorption, Decepshun became a lot more independent of Moordryd, more so than normal Psi-Class Dragons and became more strategically intelligent, performing complex moves whilst racing that even Moordryd could not do.
Other bone-marks not entirely revealed in the series were those belonging to the original Energy Booster, Fire Booster and Power Booster dragons (Ponderrah, Iympetus and Duryss respectively), who Connor Penn found and use to create Beaucephalis's pure gold draconium energy. Details of this were revealed in Episode 0: The Alchemist.
It is also possible to a certain extent to create an artificial "bone-mark"- this has only been achieved once by Word Paynn. Put together using shards of crystalline draconium from all different colours, this Chromatic bone-mark was then powered up through the combined energies of different coloured dragons. This bone-mark obviously didn't contain any possessive spirit, but the overwhelming power had an effect on the dragon it was attached to, changing its physical form and making it rampage. However, not being as powerful as a real bone-mark, this one was easily detachable from the host dragon and equally as easily destroyed.