The diamond tool is found in the toolbar (along with all the other shapes such as the cog and corner ). Click on that and you will see the options will appear along the top
There are not a huge number of options : midpoint and mirror. They are available along the top bar of the application as well as an interactive control. There are the usual options of fill color and stroke / stroke weight / dash etc. The midpoint for the midpoint (only displayed when selected) can be lifted up or down and if you stretch it far enough the design becomes a triangle.
You can fill the artwork with a solid color as well as selecting a swatch (gradients etc). You can also fill the shape with a gradient by using the gradient and that means radial, ellipse etc as well as bitmap so you can also quickly fill the design with a tiled image which can be scaled and rotated as required. The shape can also have a stroke which can be filled with a solid color as well as a gradient (apply via the swatches panel)
You can also fill the design with all kinds of additional fills by selecting some paths and then selecting the artwork and edit menu and paste inside command and those vector designs will then make up the fill. You can then go to the layers panel and manipulate the individual paths of that design. The design can be grouped in layers in many different ways
Select the current design and then go to the little gear settings button along the top of the application and click to display the current presets. Go to the right side menu and create a preset. The preset will only be added if it is different from the previous presets and nor does it record the scale or rotation, it only stores the settings (midpoint). You can use presets to quickly select a design and then add it to the current art board.
You can add a quick brush stroke using the current stroke color by going to the brushes panel (view menu and studio and brushes) and selecting a brush. The brush will then be added to the stroke. You can modify the brush even further by using the stroke settings and profiles.
You can also use the shape as a wonderful container for text. You can turn it in a text frame or text path via the layer menu commands. The shape is still live so you can still modify the midpoint as well as scale and rotation etc https://youtu.be/VUA-0v7YRgA
You can also create fascinating contoured artwork by using the contour feature. With the art selected, select the contour and drag in or out and it will generate a contour around the current selected shape and you can vary the type and also you can bake (or freeze) the appearance. You can re-apply it multiple times to create ever complex edges to your design.
Create your design and if you wish to store it away in the symbols panel then simply go to that panel (view menu and studio) and with the art selected, click the create button. You can now re-apply the design over and over and any change to the source design will be reflected in the instances.
You can create one design but you can repeat the design over and over, perhaps on top of the original design but smaller. You can also add other designs such as an ellipse or cog / gear design etc. You can select all those shapes and then go to the layer menu and geometry and use options such as subtract or XOR etc and create even more unique designs. Note that the live features are lost
You can select the artwork and then right click and select the convert to curves and the interactive design will no longer be live but you will be able to change the position of the individual nodes by using the node to create all kinds of unique artworks. This works in the same way as Affinity Photo
You can use the corner to round the corners of the artwork, though as with the node, the shape is then no longer live. A variety of corner types are available and you can create many fascinating designs by using this feature of the application. You can use it to curve one corner or all the corners of the artwork. You can also to create straight or cut designs and more.
You can also add layer effects to the diamond designs. You can find the bevel, shadow, 3D etc effects via the layer effects in the layer menu. You can also fill the artwork with a gradient with multiple stops and then add the layer effects via the fx button at the bottom of the layers panel and then use an overlay there made from multiple stops as well as using a blend mode of divide and set it to elliptical and modify the scale X and Y as well as offset X and Y and angle as well as change the stops. Both are still live and can be changed in countless ways
Create your shape, set the fill and stroke and then go to the contour. Set type to miter and set the mode to force open and then drag out the cursor and release or use the radius setting such as 37px etc. Once you have your artwork, go to the bake appearance button and click that to have a hollow filled vector that can be used with styles and more. It should be noted that for some weird reason, repeat applies of the contour do not often result in a vary pleasing vector design.
The application doesn't make it as easy as Photo to create a brush from a selected artwork. Select the artwork and then go to the file menu and export command and then select 'selection only' in the panel and save it to a PNG format file. Back in the application, go the brushes panel and then right side menu and new textured image brush and import that in. You will see the entry in the panel and you can double click it to edit it, you can also change the color to red, green etc to tint the stroke. Apply as required