Detailed nature of various dust particles
Man made components are easily recognizable due to the fact that they are mostly synthetic in nature. For example this rod shaped particle indicates that it is made of silicon that probably forms the composite material used to strengthen the body. These materials are often used to reduce the weight of the camera by adding to the strength needed without using heavier metallic components. This material is composed primarily of silicon with a high degree of hardness. When it falls on the sensor, anything that squeezes this onto the sensor will result in a scratch.
Silicon or Quartz sample:
Usually metals are softer in nature than silicon and quartz. In the following pictures one can see the different elements that make up the nature of these dust particles which are composite materials from the within the body of the camera that come off and fall on the sensor. Those that have silicon compositions are more dangerous in causing scratches as they have a high degree of hardness.
These sources could originate from the chips coming off the silicon oxide part of the sensor or mirror, electronic components or the composite that makes up the body structures of the camera, sand with quartz components and other unknown sources. In order to be safe it is highly recommended to use the sensor brush prior to a wet cleaning.