A cameo is a small portrait of an individual, always in profile, carved into layers of stone of two different colors. More exotic materials, such as gemstones and shells, were also sometimes used. They were quite expensive, and were often passed down as heirlooms.
See:
Cameo,
engraved gemA bust is a sculpture of merely the subject's head, or head & shoulders, rather than the whole body. It can be made of any material appropriate for statues or figurines, using the corresponding labor. Lacking limbs, it cannot be "raising a masterful bone bolt" or "making a plaintive gesture"--it can, instead, have an
expression, showing one of the wide array of dwarven emotions, and most likely reflecting one of the familiar set of historical events.
See:
BustA niche is a small recess set into a wall, at about shoulder height. (If it goes all the way to the floor & is at least big enough to stand in, it's called an alcove.) It is used to display a single medium-sized object while keeping it out of the way, and (if outdoors) relatively out of the weather.
What would they add to the game? How are they an improvement on statues, figurines, and pedestals?
Because busts & cameos are always (admittedly, there are real-world exceptions, but they are very few) portraits of a single individual, players can
specify in advance what they want the artwork to portray, without being slapped with a gigantic wall of text--it's just a list of names, likely broken down (or color-coded) into sub-lists like the population of the fort (living), the population of the fort (dead), gods & historical figures (your civ), and gods & historical figures (other civs). Each name only appears
once, none of those "portrait of a dwarf and dwarves" or "duel between X the human and Y the night creature" variations.
In addition, busts would form a "middle ground of respectability." If you want to decorate a dwarf's quarters (or tomb) with an image of the occupant, a figurine might seem too diminutive and unworthy of the dwarf's achievements--while a full-sized statue would be too ostentatious. Goldilocks players would be happy to have an in-between option.
In contrast, cameos are small enough to be worn as a necklace, or even a ring. Dwarven stoneworking skills would enable their production to be relatively commonplace, enough so that an average dwarf could expect to see one carved into his/her own likeness at least once in their lifetime. This combination of being intimate and personal makes the cameo an ideal token to give as a declaration of love . . . the recipient can even then use the cameo as a gem, to adorn a piece of jewelry or other item, and gift it back to the original owner.
Niches, meanwhile, are mainly space-savers, acting as a way to decorate your walls (with things other than engravings) without cluttering up traffic flow. Until we gain the ability to
control the content of engravings, niches give us the best way to micro-manage our decorations, by letting us
choose whether we want the niche to display a candle, or a totem, or a specific book, or a figurine, or a helmet, etc. Basically, it's the mechanic of building/carving a fortification, with the functionality of a pedestal.
Variations on this would include a) carving shelves/bookcases into an existing wall, and b) carving a niche
all the way through a wall, creating a small pass-through window.