The spheres cannot be created by artificial means. Raw spheres are created in a handful of coral reefs on Thessia, where environmental factors are exactly right to allow certain species of biotic corals to be influenced by tidal forces in such a way as to form the unique metacrystal structure. These spheres, when found and properly extracted from their coral surrounds, must then be refined by artisans before they can be used. Due to the difficulty of the refining process, typically only Matriarchs who have made extensive studies of the biotic fashioning process are capable of properly refining a raw sphere. A significant element of chance is involved both in the spheres' creation in nature, and in refining them, resulting in spheres being rare and highly prized. The sale of a sphere is usually only possible to a buyer endorsed by a Matriarch themselves.
When properly refined, a memorial sphere's inner structure responds to the electrochemical processes asari use to synchronise their nervous systems with their partners - in effect, a limited meld with the sphere can be undertaken, during which portions of it will be influenced by the melder's thoughts. The traditional purpose of memorial spheres is to be imbued with thoughts and memories of a specific individual, which allows an impression of that individual to be preserved, and accessed by any asari using low-level melding techniques. Imbuing a sphere is a lengthy and difficult process - although not so much so as refining it to begin with, imbuing requires intense concentration and a highly developed talent for melding and meditative techniques.
The crystallised coral substance making up memorial spheres is extraordinarily resilient, and if properly imbued, the impressions contained within them do not degrade over time. A number of memorial spheres have been preserved from the pre-technological tribal era of Thessia, and the earliest are thought to predate the asari species' transition from acquatic to land-based civilisation; the impressions imbued into the spheres by these ancient asari are still clear.
Among the possessions Samara relinquished when she became a Justicar was a "memorial sphere (bondmate)"; beyond this reference, the nature of the spheres is entirely conjectural.