On a more serious note, granular crystal would work fine. I like the idea of having small clusters of rock crystal spread through it too.
I wonder if there would be any way to do a geode when the underground diversity stuff comes in next version.
'Normal' geodes are already possible with smelter reactions. At least as stones that you can crack open for gems.
I've been using this for a while, actually.
[MATGLOSS_STONE:GEODE]
[NAME:geode]
[COLOR:6:7:0]
[TILE:233]
[ITEM_SYMBOL:15]
[VALUE:2]
[ENVIRONMENT:IGNEOUS_EXTRUSIVE:CLUSTER_ONE:90]
[ENVIRONMENT:IGNEOUS_INTRUSIVE:CLUSTER_ONE:90]
[ENVIRONMENT:SEDIMENTARY:CLUSTER_ONE:90]
[REACTION:GEODE_EXT]
[NAME:extract from geode]
[SMELTER]
[REAGENT:1:STONE:NO_SUBTYPE:STONE:GEODE]
[PRODUCT:40:1:ROUGH:NO_SUBTYPE:STONE:CRYSTAL_ROCK]
[PRODUCT:20:1:STONE:NO_SUBTYPE:STONE:CALCITE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:FORTIFICATION AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:DENDRITIC AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:PLUME AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:FIRE AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:TUBE AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:BANDED AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:MOSS AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:LACE AGATE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:ONYX]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:SARDONYX]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:AMETHYST]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:CHRYSOPRASE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:PRASE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:CARNELIAN]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:YELLOW JASPER]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:PICTURE JASPER]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:BROWN JASPER]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:SMOKY QUARTZ]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:QUARTZ_ROSE]
[PRODUCT:2:1:ROUGH:NO_SUBTYPE:STONE:MILK QUARTZ]
Granted, most of what I know of geodes comes from Wikipedia and mindat, so it's most likely not an accurate depiction of the contents of a geode. It is, however, proof of concept.