Dolomite aka Ground Magnesium Limestone aka GML

Dolomite is composed of calcium magnesium carbonate, ideally CaMg(CO3)2. Dolomite and dolomitic limestone are added to soils and soilless potting mixes as a pH buffer and as a magnesium source.

What it is used for

Magnesium  increases chlorophyll production in leaves thus enhances the green colour of the leaves. 

Some common magnesium deficiency symptoms are a characteristic inter veinal chlorosis pattern where the veins remain green but tissues between the veins turn yellow, leaf curling, stunted growth, and lack of sweetness in the fruit.

How to Apply

If you're adding it to an empty garden where you plan to plant flowers or vegetables, turn it into the top 6 inches of the soil before you plant. To change the soil's pH, determine how much you need and spread it on top of the soil. For example, if your soil has a pH of 5.5 and you want to raise it to closer to 6.5, add 10 pounds of dolomite per 100 square feet of space. That would be equivalent to a 10-by-10-foot bed or a 5-by-20-foot bed. To prevent burning existing plants, spread the applications out throughout the growing season. Best applied before planting.

Get yourself a PH Meter, which readily available on most online shops. It does comes handy.

More about Dolomite:

https://geology.com/minerals/dolomite.shtml