Register a geode:

geodes_lib:register_geode({
    wherein = node in which geodes generate,
    y_min = minimum level at which geodes generate,
    y_max = maximum level at which geodes generate,
    scarcity = rarity of geodes,
    core = node comprising innermost part of geode,
    core_alt = replacement for node comprising innermost part of geode,
    core_alt_chance = chance of core_alt replacing core,
    shell = array of nodes comprising geode shell, from outermost to innermost,
    radius_min = minimum radius of geode cavity,
    radius_max = maximum radius of geode cavity,
})

See amethyst mod for an example