The fantasy setting has (the typical) medieval-level ordinary technology with magic use being a rare craft (although few are completely insensitive to magic — theoretically able to use it — most have limited aptitude and far fewer have any training) of significant but not enormous power (no equivalent of nuclear weapons but teleporting "gates" are not rare and magic fertilization exists).
Adamant is a stone-like material that is literally unbreakable (under normal conditions). Its native form is a sphere, typically about a millimeter in diameter. Under very high temperatures (achieved somewhat readily by skilled magicians) it becomes malleable and can be merged into larger units merely by contact. Under extremely high temperatures (achievable only by one group of magicians according to aptitude and skill) a larger unit can be separated by high tension across an already thinned (under very high temperature) section, but this technique is rarely used (with native pieces being small).
Adamant is rarer than gold (at least on the continent on which the story is primarily set) and processing is relatively labor-intensive and skill-constrained, so common items would not be made from adamant. Being extremely difficult to adulterate (it is effectively immiscible and making a sphere with an interior of base material would be expensive) and easy to test (besides being unbreakable, adamant has an extraordinarily smooth surface and obsidian-like color and luster). This implies that it could be used as money (store of value, medium of exchange), though small fixed weight units would be impractical (and large chunks are less valuable by weight because separation is so difficult). Its rarity and luster would also admit use as a native (spherical) gemstone or artificially flattened form (both increasing the investment/value and allowing a larger visible surface). Adamant's density will probably be around that of quartz; certainly much less dense than iron and much more dense than water.
However, I want adamant to be put to other uses, despite its rarity. It is used for weapons, both because of its durability (forming a very sharp edge or point is difficult — adamant has something like extreme surface tension and adamant 'foil' is practically impossible to make — but once cooled adamant is solid and an edge would retain sharpness) and interaction with magic — is it almost entirely "magically neutral" but increases magical affinity of nearby and touching materials (especially for types of magic harmonious with adamant). Its influence on magical affinity also makes it useful in enchanted items more generally, but the enhancing effect is mainly worthwhile at the higher end of enchanting effort where a more-or-less fixed proportion augmentation has greater value.
Outside of money, decoration, (rare) weapons and armor-parts, and magical items, what would adamant be used for?