The selenium toner contains sodium selenite (Na2SeO3) as one of its active ingredients, which is the source of the selenide (Se2−) anion combining with the silver in the toning process.

[6] The crystal structure of this polymorph is highly related to the acanthite phase of silver sulfide.

For Ag2Se, this polymorph is increasingly unstable for larger crystallites, which explains its absence on the bulk phase diagram.

[7] It is thought that the influence of the surface energy plays a role stabilizing this phase on the nanoscale, thus allowing its experimental isolation.

Electronic structure calculations for this polymorph of Ag2Se suggest that it is a narrow-gap semiconductor,[6] which is supported by experimental evidence as well.