Gravel Recycler
Gravel Recycler is an artifact for Isaac that gives you 1 Temp Shield every time an Asteroid is destroyed for any reason.
Clarification
Geodes count as asteroids for the purpose of this artifact, as Books will helpfully point out if she's on your crew!
You immediately get the Temp Shield when the asteroid is destroyed. If it's destroyed during the enemy's turn, you'll get the Temp Shield during the enemy's turn, meaning it can protect you from future attacks the enemy does during their turn. (Remember that enemies attack from left to right, so an enemy's shot on an asteroid can protect you from an enemy shot on your ship to the right of said asteroid.) But as usual, your Temp Shield goes away at the beginning of your turn.
This applies to asteroids that you release due to cards / Rock Factory too - if they're destroyed immediately because you're deploying them into a midrow object, this will trigger. (This means you can get 2 Temp Shield by deploying an asteroid into another asteroid, destroying them both - which will likely happen if you have 2+ stacks of Rock Factory).
Strategy
Any build that regularly defends with asteroids can be boosted to the point of being difficult to damage with this. Keep in mind that enemies fire from left to right, so in order to use the Temp Shield that this provides effectively, cover the left side of your ship with asteroids, so that enemies will destroy asteroids before striking your ship directly, allowing you to benefit from the Temp Shield. Similarly, you'll tend to want to dodge to the right. The Artemis and Tiderunner conveniently have a left-side Missile Bay and a cannon to its right, and the Gemini can deploy left-side if you're using it right. The Ares can also pull this off with a little help from Droneshift and Evade thanks to its small size and right cannon. The Jupiter probably shouldn't be playing asteroid builds in the first place, as Drake points out.
It also making mass-deploying asteroids much more palatable. Normally, deploying an asteroid into an asteroid loses both asteroids, which is a fair bit of sunk value, but this artifact rewards you with 2 Temp Shield for that situation. In particular, with this artifact, having more than 1 stack of Rock Factory will provide a steady source of Temp Shield, which in conjunction with other defense can make it difficult for enemies to hit you. Large Boulders can provide an impenetrable defense. (Just remember that you do need a plan to damage the enemy despite all these asteroids.)