Another more science fantasy approach: If a dwarf can earn a dwarf-weight of gold, he can petition the priests of the life-forge for the right to craft an heir. It starts with a grand ceremony, where priests clad in white robes and coats lead the way through the warrens, swaying and chanting the ancient words passed down from master to apprentice. As they lead the way, the priests periodically stop to brandish the coins and wrought jewelry the father has earned, seeking the favor of the spirits of the breathing mountain. Eventually, following a long, winding path, they reach the vast chamber of the heart. As the entire community watches with heat-flushed faces, the priests cast the offering of gold into the crucible, where it melts down into a bubbling pool
Then using the rote rituals of their caste, which have been drilled into them from childhood, the priests perform the dance of birth. They move gracefully from station to station, stroking and manipulating the strange sculptures and carvings. In response, lights start to dance across the room, and ancient sounds of clanking and whirring emerge from the earth. The molten gold sluices along channels, and into one of the molds. After a time of cooling, the mold is broken, and with great pincers and an iron stretcher, they carry the still hot but firm shape to one of the vats, and cast it in with a burst of steam and sacred cries. The assemblage of faithful below, staring through the windows in the vat, gasp as they see the form captured by a net, which cradles and slowly swaddles the vaguely humanoid golden mass, suspending it in the liquid. Eventually the dwarven witnesses shuffle out of the room, in pairs, leaving the priests behind at their stations, and the soon-to-father kneeling in prayer before the vat.
Weeks pass. The petitioner remains at his post in front of the vat. Occasionally a young priest offers a bulb full of liquid to the potential father, sustaining him. He may become delirious or ecstatic; if necessary, he will be restrained by the attentive priests, with chains bolted to the floor. As time passes, the priests introduce new materials into the vat. Layer after layer builds up, hiding the glossy golden heart with coatings, woven strands, and odd little implants. Eventually, a recognizably dwarven shape starts to emerge. The final layer is the skin, rough, dark, and hardy, as dwarves should be. At this point, a bell chimes, resonating through the warrens. Those closest to the new father drop what they're doing, and race to the birth chamber. Others finish what they're doing, and put away their tools. But except for a few guards, eventually the entire community filters in.
They wait in silence. Eventually there's a gurgle and a rush, and the crowd gasps as the vat starts to empty. The net falls flat, and the body within slumps to the floor. The father-to-be-no-longer dashes to the door, or if restrained, starts to pull against the chains. The priests gently release him if necessary, and then with a spin and a quick dance, they open the door. The new father surges inside, and cradles his new son. The crowd watches in silent reverence. There may be tears, there may be words or just noises, but that is as it should be.
The dwarfchild is dried with towels, draped in a gown, and placed on a stretcher. Then a new procession takes the father and son to their home warren, and leaves them. The son will wake in a few days, but that is a personal matter. Only the father and a senior priest attend.
Gold is malleable, bright and beautiful, and never tarnishes. Dwarves are proud of their heart, and believe other races are made of inferior metals.