Black seed (Nigella sativa) and cinnamon (Cinnamomum zeylanicum) are not listed as major anti-diabetic herbs in Ayurveda research literature. Cinnamomum tamala (common name tejpata, malabathrum) finds mention. The main medicinal action of the plant is its ability to reduce blood sugar. Experiments on rats validate this property. The effect is comparable to the antidiabetic modern drug Glibenclamide (Chakraborty et al., 2010). These studies raise the likelihood of its probable use as an adjunct medicine for treating diabetes (Kumar et al., 2012).
If you have heard about black seed and cinnamon as anti-diabetic plants, then this could be a traditional use that is not yet adequately studied by scientists and reported as an anti-diabetic. This is quite common.
Many anti-diabetic formulations sold in the market contain plants and ingredients whose effectiveness is not supported by credible scientific literature.
In my research, I have found the following four herbs to have the maximum scientific support as anti-diabetic. The four herbs are:
Gymnema sylvestre, Trigonella foenum-graecum, Eugenia jambolana, and Curcuma longa. There are additional herbs from Chinese medicine that have scientific evidence in support of their anti-diabetic properties also.
Many popular and even Ayurveda experts make claims supporting particular formulation effectiveness as an effective anti-diabetic formulation. Readers should be wary of these claims and look for published scientific support before accepting these claims.
Additional reading reference:
Natural Solutions for Diabetes