I’m about to try my most complex project, carving into a piece of rock from Mount Etna. It’s lava rock, quite brittle but strikes me as not too much of an issue to cut. Any speed and feeds recommendations for this or similar rocks?
I was thinking of trying the Limestone settings from the wiki https://wiki.shapeoko.com/index.php/Materials#Shapeoko_3
No recommendation for cutting your rock but be sure to wear eye protection. As you said the lava rock is brittle and it may explode when you try to tool it It may cut well but prepare for the worst and hope for the best.
I worked with my mom several years back on a rock project she had. I ended up using diamond bits to do anything. Any other bit just “melted” away.
Here is a pretty good instructable, that shows the type of bit to use and also suggests using water to keep the bit cool.
There is a Shapeoko user that has come up with an add-on accessory for engraving.
Good luck, and keep us up to date on how this project goes.
Here’s my thread regarding using PCD (poly-crystalline diamond) bits to carve marble tiles for coasters. I feel like I am spamming this link, but I haven’t seen any other posts regarding carving stone (vs engraving stone), so here it is again. 
I bought diamond bits from AliExpress. They took 3ish weeks to arrive, but were ~$12.00USD each vs ~$175.00USD for the Amana bits from sites like ToolsToday.com.
Good luck and please report back with pics and any additional info you can think of. Things that worked well are nice to hear. Things that went bad are even better to hear about so the next person doesn’t make that mistake as well.
This is something that I’ve been thinking about lately. So I’d be very interested in following your development.
You may want to consider these bits https://m.aliexpress.com/item/32964230930.html
Thanks for all the replies. It’s certainly not similar to marble, much softer, going to try normal bits. Fingers crossed. Will post photos soon.
It was successful. I followed the limestone speeds and feeds, was more cautious and did 2mm roughing step down not 4mm. Using the 201 bit.
Unsurprisingly it did crumble in a few places, but with the rock I have that was always going to happen.
Before and after:
This one screams to be made into the geo shape of the volcano
Maybe you could carve a mold and pour in some hot lava.
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