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Color Streak
Looking at the color of the stone is not the best way to identify a gem, because some colors are artificial or false. But if you rub a gem against a flat stone or ceramic tile, a streak of color will be left behind. For example, even though hermatite's luster is black, the streak it leaves behind is red. See the Resource article below for further reading.
Crystal Shape
The morphology, or shape, of a gem helps determine its identification by looking at the crystal symmetry. Some gems have hexagonal (six) prisms, and others are octahedrons (eight). These shaped numbers represent the atomic arrangement of a gem. If the gem is already cut, it's much more difficult to determine its identity by looking at its shape.
Hardness
All minerals have a different value of hardness, known as the mohs hardness scale. From most to least hard, here is the order of minerals with their mohs number: diamond--10, corundum--9, topaz--8, quartz--7, orthoclase--6, apatite--5, fluorite--4, calcite--3, gypsum--2, and talc--1. To test the hardness, scratch the minerals to see which are weaker or stronger. This is a destructive test, but is diagnostic. See the Reference links below for further reading.