A wholesale jewelry brand sends over a wax tree carrying 8 mm faceted amethysts in half-bezel pendants. The first casting run comes back with three cracked gems inside the metal, and the wax room swears the stones were seated before investment. The failure is not the wax. It is the sprue: a single 3 mm ingate into a heavy stone pocket lets the metal hit the gem at pouring temperature, and the thermal shock splits the pavilion before the flask even cools. Spruing large stones on a lost-wax tree is a separate skill from spruing plain metal waxes, and the numbers change with every carat of gem you hang on the tree.
Why large stones crack on the tree
Colored stones and many transparent synthetics cannot take the burnout cycle of a standard gypsum flask. Quartz, amethyst, citrine, and garnet crack from thermal shock above roughly 550 C (1020 F); opal, turquoise, and pearls cannot go in a burnout kiln at all. Even when the stone is rated for the cycle, the sprue geometry decides where the incoming metal lands. A thin ingate aimed directly at the pavilion acts like a flame thrower, pouring 700 C silver or 900 C 14K gold onto a stone that sat at 550 C for two hours. The 150 to 350 C delta cracks the facet and sometimes the girdle. The Lost-wax casting reference on Wikipedia covers the flask physics, but it does not tell you where to point the metal stream.
Sprue diameter and ingate angle for stone waxes
We run a 4 to 5 mm sprue for any wax carrying a stone larger than 6 mm, and we angle the ingate so metal flows into the bezel pocket from the side, never straight at the pavilion. The side entry spreads the stream across the bezel wall before it reaches the gem, dropping the effective contact temperature by an estimated 80 to 120 C. For stones above 10 mm we add a secondary 2 mm vent sprue on the opposite side of the pocket so air and wax gas escape instead of pushing back against the incoming metal. That vent is also where the first slugs of cool metal land, which keeps the hottest stream off the stone.
Flask placement and tree density
A stone-laden wax tree goes into the middle of the flask, not the rim. The flask wall runs hotter during burnout and cools slower during pour, which is exactly the temperature gradient that stresses a gem. Leave a 20 mm investment wall on every side and do not stack stone waxes more than two high in a 3-inch flask. Our write-up on sprue design for jewelry casting trees covers the geometry on plain waxes, and sprue and investment defects shows what a too-thin ingate looks like after shakeout. When stones are on the tree, we also drop the slurry water ratio by 1 mL per 100 g powder so the mold wall is denser and breathes slower.
Bench numbers we hold
Sprue diameter: 4 mm for 6 to 10 mm stones, 5 mm above 10 mm. Ingate angle: 30 to 45 degrees off the pavilion axis. Vent sprue: one per pocket above 8 mm. Flask hold temperature: 500 C (930 F) instead of 730 C peak, with a 90-minute soak only. Tree count: no more than 14 stone pieces per 3-inch flask. We photograph every stone-wax tree before investment and tag it with the stone material so the caster knows which flask to pull early. The connection between tree piece count and layout is tighter when gems are involved, because one cracked stone scrapes the whole run.
FAQ: Spruing large stones in investment casting
Which stones can actually go through burnout?
Only sapphire, ruby, and some synthetic spinel and YAG handle a full 730 C cycle. Quartz-family stones (amethyst, citrine, ametrine) survive a reduced 500 C cycle if the ramp is slow at 1 C per minute. Opal, pearl, turquoise, emerald, and tanzanite come out after the metal and are set by hand.
How thick should the sprue be for a 10 mm stone?
Use a 5 mm main sprue with a 2 mm side vent. A 3 mm ingate under-feeds the bezel pocket and causes shrinkage porosity around the setting; a 6 mm ingate wastes metal and increases thermal mass on the pocket.
Does flask rotation or angling matter?
Yes for tilt-pour machines. We angle the flask 25 to 30 degrees so metal climbs the sprue rather than dropping straight into the pocket. For vacuum-pressure casting, the flask stays vertical but the ingate still points sideways.
What if a stone cracks after the first pour?
Pull the tree, drop the peak hold by 30 C, add a 2 mm vent, and slow the ramp from 5 C/min to 3 C/min between 300 and 500 C. Do not raise the pour temperature to compensate; that widens the crack.
Send over your stone-wax tree for a sprue review before you pour. Our custom casting bench runs gypsum-bonded investment in 1 kg to 50 kg batches, photographs every stone-laden tree, and tags flasks with gem material and reduced burnout schedule. Small-batch OEM custom manufacturing starts at 30 pieces per design; send the waxes and we will quote the tree.
Frequently Asked Questions
How do I verify a factory before placing a bulk order?
Request a production video, business license, export records and a paid sample run. A factory that accepts a 50-piece test order is usually a real manufacturer.
What payment terms do you offer?
30% T/T deposit with the balance against the bill of lading. This protects both sides on first orders and can move to net terms after a track record.
What is your normal reject rate?
3–5% under 10x loupe inspection for cast and set pieces. Defective units are replaced free in the next shipment.
Where is the factory located?
Wuzhou, Guangxi, China—the core lab-grown and moissanite manufacturing cluster. Shipments go via Guangzhou or Shenzhen ports.
