Not all blacks are heavy. Some shades of black create a sense of closure. But there are also shades of black that make an item deeper, quieter, and more substantial.
Onyx does not shine in an obvious way. The black of Onyx is quieter. It lies on the silver surface like a deliberate dark area, small enough not to overpower the design, yet deep enough to change the entire feel of the item.

Onyx does not soften silver. Onyx mutes silver.
Some stones are placed on jewelry to make the item more brilliant. Onyx is different.
Onyx does not move silver towards a brighter tone, but pulls it down to a deeper, more muted note. The black of Onyx makes silver less flat, less "clean" in the sense of being too new, less purely decorative. When paired with S925 silver, Onyx creates a feeling closer to an object with depth than a simple accessory.
It doesn't shine. It retains light.

Black Sun
This is not a sun of brilliant light.
In alchemy, this image is often associated with nigredo - the "blackening" stage, the disintegration and dissolution of the old before it can transform into a new state. It is the first, heaviest, darkest, but also the most essential stage of the transformative process.
If understood symbolically, the black sun does not speak of darkness as an end point. It speaks of darkness as the beginning of transformation. Decay to prepare for rebirth.
There are periods when people are not bright. Not clear. Not in their best state. There are times when all the old things inside must be disassembled, must settle down, must lose their original form before a new structure can emerge.
That is a very different kind of darkness. It becomes a sign of compressed inner strength, of growth happening in silence, of a part of a person that does not need to be displayed but still exists very clearly.
Not all men choose to walk in the light. Some choose to walk in the night. Not because they are lost, but because they believe they are strong enough to find light from within.

Not all light needs to be dazzling.
HELIOS does not view light as something that must always shine brightly.
Some light lies on the silver surface. Some light only appears when the item moves. Some light is held in a subdued area, in a deep crevice, in a black stone that does not reflect too much outward.
Onyx gives HELIOS a different way of speaking about light. Not the light of display, but the light after it has passed through darkness.
