Sapphire

Blue gems show up embedded in stone through the underground and cavern layers, and they read as dark patches unless something is lighting them properly. A Spelunker Potion turns a wander through a cave into a full sweep. There are also gem caves, either a chamber full of a single type or a mixed one holding everything except amber, and stumbling into a sapphire one hands you more than you will realistically spend.

Hand mining is the slow route. An Extractinator converts silt, slush and desert fossil into gems, which means every mining trip already produces some without you going looking, so hauling that material home is worth the inventory space. Geodes in the underground desert break open into gems too. The permanent fix is a Sapphire Gemcorn planted in the caverns: it grows into a tree you can cut and regrow indefinitely, at which point the gem stops being a resource and becomes a crop.

Blue sits low in the gem order, so nothing that asks for sapphire justifies a dedicated expedition. Collect them while digging for something else and you will have plenty by the time a recipe wants them. The exception is gemspark blocks, where lighting a whole build eats through a stack faster than anyone expects, and the Rainbow Phaseblade, which takes a handful and then leaves you with a pile you will never touch again.

Gems place like any other block and a mounted sapphire sparkles without a light behind it, giving you illumination and decoration in the same tile. If you want the color as bulk building material instead, a Heavy Assembler in a Graveyard turns them into Sapphire Stone Block, which is the version to use when you are covering a wall rather than accenting one.