To create a diamond symbol in Illustrator, go to the rectangle tool and create a square by holding down the shift key and then rotate it by 45 degrees and then you can use the direct selection tool in the tools panel to select the top point and drag upwards with the direct selection tool. Go to the bottom point and drag downwards to create the classic form. Once you have done that, you can fill it with different colors perhaps go to the top left with the selection tool selected or the rectangle tool selected or use the properties panel to re-color the design with a gradient. You can change the color of the design using the gradient tool in countless ways or apply a swatch if you have a selection of colorful tiles in there. You can also create multiple diamond shapes by duplicating the design by holding down the alt or option key and then combine them to make your symbol. You can also add effects to it such as warping the shape or applying Photoshop effects to the symbol. You can then go to the symbols panel found in the window menu and then goto the right side menu and select the create command to add it to the panel and the design can then be used in future work. Clearly, this is a start point, you can create millions of variations of this artwork such as filling it with different shapes, using the curvature tool to manipulate it further as well as adding brushes as a stroke to the design and much more.