There are crystals in it

The four

Graining has four causes and they are worth separating because only one of them can be fixed while the pan is on the stove. The other three are decided earlier, and a batch that grains cannot be rescued by cooking it further.

• Excess sugar. More than the solution will hold, so it comes back out.
• Undissolved sugar sticking to the sides of the pot. It falls back in as a seed crystal late in the cook.
• Tartrate crystals in grape juice. Specific to grape, and nothing to do with your sugar.
• A mixture cooked too slowly or too long.

The one you can prevent on the day

This is the one worth building a habit around, because it is entirely mechanical and costs nothing. It also happens quietly, which is why it is better prevented than watched for.

Sugar on the sides of the pot. It is dry, it is hot, and when it drops back in it gives the syrup something to crystallize around. Wipe the inside wall down before the mixture comes to a boil.

Grape is a different problem

Tartrates are naturally present and will form in grape juice on standing. Letting the juice stand cold and decanting off the clear part before you cook is the handling that addresses it, and it is a juice step rather than a jam step.

What a customer thinks it is

Crystals are harmless and nobody believes that, which makes this a commercial problem rather than a safety one. A gritty jar reads as spoiled or as carelessly made, and no label explains it away. That is reason enough to treat it as a defect even though it is not one. A jar returned for grit costs more than the batch it came from.

Sugar, almost always, and therefore cheapness. The crystals are visible through glass, they feel wrong on the tongue, and they cost a repeat sale without ever being reported as a complaint.

References