&gt; The article says &quot;emits a soft glow&quot;. Since that&#x27;s impossible for something which isn&#x27;t radioactive…<p>Not at all true.<p>In this case, it could be a phosphorescence or fluorescence process (the NYT says fluorescence), initiated by ambient light.<p>I skimmed both scholarly publications on this material but didn&#x27;t find any mention of optical properties.  I would ordinarily guess that any optical emission would involve an internal defect site (as happens with other optically-active diamond structures), but I&#x27;m thinking that&#x27;s not a good guess given the synthesis process and overall quality of the monocrystalline product.  A better guess might be some sort of &quot;deep trap&quot; emission involving surface defects.