Alright, if you’re going to grow quartz in a jar, you’re going to need a lot of heat. Glass won’t survive the 1200‑1700 °C you need, so swap that jar for a small metal or ceramic crucible. Here’s the straight‑up plan: 1. Take a clean, high‑purity silica powder or a piece of pure quartz as a seed crystal. 2. Put it in the crucible and close the lid tightly. 3. Heat it in a furnace or on a propane torch that can reach at least 1200 °C. 4. Raise the temperature slowly – about 5–10 °C per minute – to avoid thermal shock. 5. Hold at that temperature for a few hours so the melt can feed the seed. 6. Turn off the heat and let the crucible cool very slowly, ideally at 1–2 °C per hour, until you see a crystal forming. No fancy gadgets, just a good seal and a steady supply of heat. If you can’t get to those temperatures, you’re out of luck – quartz isn’t going to grow in a cold glass jar. That’s the practical truth.

Use a lid that actually fits. I’d go with a small metal cup that has a threaded top – screw it on so the threads hold it in place, then put a rubber or silicone gasket in the rim to keep the seal. If the metal expands, the threads will just pull it tighter, not let it pop off. Add a tiny vent hole or a pressure relief screw so any trapped gas can escape; that keeps the lid from getting stuck. Keep the lid snug, preheat it a bit before you seal, and you’ll avoid that “cracked‑off‑lid” problem. Simple, no fuss.

Good plan. Just remember: heat the lid separately first, note the temperature where it starts to warp, then tighten the threads. Keep that vent screw loose until you’re sure the pressure stays low. Once the whole thing’s sealed, run the slow cool and keep an eye on the crystal’s growth. You’ll get a decent chunk if you keep it simple. Good luck.