These are baby cave elementals. Voidlings, absencelemenetals. Not the abstract expressions of rock, but living forms of voids within the rock. The caves. You won't meet the parents. They have strange business deep within the core, below the Mohorovic discontinuity. These are baby negaspawn, migrating through the earth and incarnating as minor absences, like gaps between your teeth or silent whiteness in the letter 'o'.
Splinterlads should be harmless, but sometimes, they are playful. This makes them deadly. They dive up out of non-ity like dolphins breaching surf curve briefly in the air. And just as dolphins sport with ships the Splinterlads love folk. A cave within the rock, a moving breathing spot within the cave. This is addictive paradox to the voidlings. Just like the ships hull and sails are to the dolphin, something within its world and without.
You must never amuse them.
Spliterlad Nymphs are the reason the physics of granular materials seems to make no regular sense. They are whimsical. The worms make worship of them. Earthworm cysts are endlessly renewing micro-temples arranged in cryptic unseen mandalas and honing strange ant-level micro-climates. The larger worms still recognise them, in particular, the Purple Worm, which often carries a murmuration of voidlings flocked around in silent converse. The spell 'rock-to-mud' is actually a decayed fragment of ancient worm-songs learnt by Neanderthal hyper-shamans and passed in damaged form to their Sapien wives.
Think Hitchcocks 'The Birds' except the birds are briefly burning bubbles under your skin, fizzing solids, bubbly rocks and stones that melt in bird-flock-curls.
The best way to avoid a Spliterlad flock it to have glass and crackable chalk. They also hate quartz.
Whirl the glass around your head like a falconers lure.
If a flock flies through you, your teeth will explode in your head, your blood vessels will burst, your bones will splinter, the fine arteries in your brain will burst. This rarely happens though. Usually just the edge of the flock will dance along your equipment and burst your ropes, crack your swords.
The secret to avoiding them is to break good things. They can talk to the calcium in your bones, so people with multiple broken bones in their past will have a bonus to reaction rolls. Break something else useful, like a relationship, a trust, a heart. They will ask you to. You can hear them whispering distantly 'break break break break break'