Json:block:particle

The torch is a good example of how to use those particles ...

	particleProperties: [
		{
			hsvaColor: [{ avg: 20, var: 20 }, { avg: 255, var: 50 }, { avg: 255, var: 50 },  { avg: 255, var: 0 }],
			gravityEffect: { avg: 0, var: 0 },
			posOffset: [ { avg: 0, var: 0.1 }, { avg: 0, var: 0 }, { avg: 0, var: 0.1 }],
			velocity: [ { avg: 0, var: 0.025 }, { avg: 0.5, var: 0.1 }, { avg: 0, var: 0.025 }],
			quantity: { avg: 0.015 },
			size: { avg: 0.5, var: 0 },
			sizeEvolve: { transform: "quadratic", factor: -0.7 },
			lifeLength: { avg: 1.5 },
			glowLevel: 64
		},
		{
			hsvaColor: [{ avg: 0, var: 0 }, { avg: 0, var: 0 }, { avg: 40, var: 30 },  { avg: 220, var: 50 }],
			opacityEvolve: { transform: "quadratic", factor: -16 },
			gravityEffect: { avg: 0, var: 0 },
			posOffset: [ { avg: 0, var: 0.1 }, { avg: 0, var: 0 }, { avg: 0, var: 0.1 }],
			velocity: [ { avg: 0, var: 0.025 }, { avg: 0.15, var: 0.1 }, { avg: 0, var: 0.025 }],
			quantity: { avg: 0.05 },
			size: { avg: 0.25, var: 0.05 },
			sizeEvolve: { transform: "linear", factor: 0.5 },
			particleModel: "Quad"
		}
	],

There is also a complete tutorial about particles, which should help you to find out what each property does.