There's an idea floating around the Orthodox world (and maybe even wider) that stones from Mount Sinai are unique and were branded by God with internal engravings of bush-like shapes in memory of the incident of the burning bush that allegedly took place there.
This site had (see update) explained the idea quite well:
"The Mysterious Rocks from Mount Sinai: No matter how you split a rock from Mount Sinai, you will see on all sides mysterious engravings of a bush. I've witnessed this personally and can testify that it's true. (this is a unique phenomena in the world) I once heard an explanation in the name of a Midrash: When G-d sent Moses to Egypt to report to the Jews about the redemption and then to come back to Mount Sinai, Moses asked G-d: 'how will I be sure which mountain to return to?' (since at first, G-d spoke to Moses only from Mount Sinai). G-d made a miracle and all the rocks from Mount Sinai became engraved with the 'burning bush'."
Then it follows with a series of images of broken stones that allegedly came from Mount Sinai, here are a couple if you're too lazy to go to the site yourself.
[Pictures removed from site.]
I don't know if that midrash is real or not, but it looks pretty impressive, eh?
Anyhow, I did some research and it turns out that that these types of mineral fomations are not uncommon at all. What happens is that usually some kind other mineral like iron oxide or manganese oxide sets into the cracks of the stones and gives this dendritic appearance.
"A crystal dendrite is a crystal that develops with a typical multi-branching tree-like form. Dendritic crystal growth is very common and illustrated by snowflake formation and frost patterns on a window. Dendritic crystallization forms a natural fractal pattern...
In paleontology, dendritic mineral crystal forms are often mistaken for fossils. These pseudofossils form as naturally occurring fissures in the rock are filled by percolating mineral solutions. They form when water rich inmanganese and iron flows along fractures and bedding planes between layers of limestone and other rock types, depositing dendritic crystals as the solution flows through."
I found some pictures on the web and I wanted to post them here, but for whatever reason Blogger is giving me issues so I'll just link to them and let you check 'em out yourself.
Here, if you scroll down about a third of the way down the page you'll see three pictures with this caption written underneath: "[Dendritic (like tree branches): quartz with black manganese dioxide crystal inclusions, sandstone matrix with iron oxide dendritic crystals on surface, dendritic native copper crytals]."
Here too, if you scroll down to the bottom, you'll see another three pictures with this written underneath: "[Dendritic agates and a dendritic sandstone piece]."
And here's a good picture of a dendritic agate from North India.
Here's a good blog post written by a geologist on dendritic formations in general.
So, as they say on MythBusters, this myth is Busted!
Update: The webmaster of the above linked website has acknowledged the error (see comments) and has removed the page from his site.