@TommyDee great to see where your VC inspired creativity is taking you. I looked up quince and all the info seems to be from knife makers. It looks very different to the stuff you are using. Not turned any myself. Most unusual 'wood' I've made a stem from was a stalk from a weed plant a friend grew. Your use of the brass tube is the method pen makers use. Personally I don't like it. To get nice slim stems you have to turn the wood very thin which leads to cracking. Ambient temps and humidity put a lot of stress on the very thin wall. I recently saw a display of pens and 3 out of 8 had cracked very badly. This was down to a warm, dry environment. Now factor in the heat generated by the tip and its a recipe for disaster. You may get lucky and have some pieces may hold up just fine, but.... The other thing is most wood blanks are cut from logs in a specific manner so as to avoid the natural shift in the wood as it dries and the change you get with the stress turning causes. If you are using branches and following the line of pith to centre the drill this will cause problems. The best thing to do is cut and season a pieces that's at least 2" or 3" diameter. Once it's dry split it into 4. Drilling the centre of one of these quarters is better and should be less prone to cracking. Have fun.