Oh ... I forgot to stay on topic!!
The strangest material I've ever made into a pen, eh? .....
Well, I've got a list of things to say about related subjects here.
Most types of crystal that are related to quartz .... are harder than glass and tend to SHATTER rather than grind down. The dust is extremely bad for eyes, mucus membranes, nose, mouth, and lungs.
That having been said, it didn't stop me from ATTEMPTING to inlay crushed amethyst into a purpleheart pen.
It tore out of the CA and got lost in my shop/living room area ... shattered into tiny bits ... and generally wasn't worth the effort I tried to put into it. Now that I have some diamond grit plates in grits between 150 and 3000, I may attempt something like this again, but I'll need a polishing compound used for actual jewelrymaking, like cesium oxide or something, for the final polish (goes up to 50,000 grit or something like that... and they charge around 50 bucks a ounce!)
Other materials I've tried and failed with .... crushed Tigereye (wanted to make that into a pen for the bash ... it just ripped off the brass barrel or shattered, much like amethyst except more fragile).
Malachite - the dust from this is poisonous, it has ribbons of lighter and darker material running through it that will tend to act as cleavage lines so the whole thing splits right along them ... ect. Sanding it down can work ... just have to be VERY patient and take great care with how much pressure you put on it with the mandrel. I give great respect to the one that put that malachite pen in the bash ... that material is fragile to work and difficult to work with!
I haven't had the time to try the lapis lazuli for making a full size pen blank ... don't have a large enough piece, anyways, but I have pieces large enough to do segmenting work with. The problem with this, however, is the dust. Just like malachite, lapis is a copper oxide, and is poisonous.
I tried once, with a piece of amber, to make a pen. Word of warning ... amber is considered a semi-precious material ... it's called "fossilized tree sap" ... but it's definitely not stone or crystal material. In fact, if you hold it in your hands long enough, it gets soft ... to the point where you can easily deform it with finger pressure!
Some of the more difficult material I have been working with lately has also been a great challenge. Crushed abalone shells cast in Alumalite. Definitely need to bake it, with this Amazing Clear Cast resin, to harden the acrylic, and a carbide tipped tool is certainly a plus ... the dust from this is harsh, but the results when it's finished can be stunning!
My next side project, however, involves taking washi (rice paper) and inking it with mokke (wood) designs, and then creating a pen using such a design wrapped around the body and sealed with CA ....
Or possibly taking the same and creating some laminated projects of it with many layers of rice paper and epoxy or alumalite resin ... similar to making micarta. I have several hundred sheets of the rice paper, so the ingredients aren't hard. (yet)