look on big map there are spikes showing on it in the hive area. there is a cluster of 4.
You need the bottom right one.
On the mini map it looks like a brown circle.
Go down the ramp to the bottom. There are 2 entrances here. Take the one furthest from the ramp. Go right at the Y. the crystal is obvious a big white spike in the middle of the room. just click to get the rubbing. No need to search bags for item to use.
hope this helps
geode druid stormscale